Magnetically steerable continuum robotic guidewires for neurovascular applications

ABSTRACT

Robotic devices and methods for performing minimally invasive procedures on the vascular system, particularly cerebrovascular and endovascular neurosurgical procedures, where a submillimeter-scale continuum robotic device is configured and adapted for active steering and navigation based on external magnetic actuation. The submillimeter-scale continuum robotic device includes an elongate body having an inner core and an outer shell, where the outer shell is fabricated of an elastomeric material having a plurality of ferromagnetic particles dispersed therein.

This invention was made with Government support under Grant No.N00014-17-1-2920 awarded by the Office of Naval Research, and Grant No.CMMI-1661627 awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention generally relates to robotic devices and methodsfor performing minimally invasive procedures on the vascular system,particularly cerebrovascular and endovascular neurosurgical procedures.More particularly, the present invention provides a submillimeter-scalecontinuum robotic device adapted for active steering and navigationbased on magnetic actuation. As such, the present invention roboticdevice is capable of navigating the considerably smaller and moretortuous vascular structures, including the microvascular system,thereby enabling the performance of minimally invasive remote procedureson these difficult to access location.

BACKGROUND OF THE INVENTION

Stroke remains one of the main causes of death and the leading cause oflong-term disabilities in the United States. Stroke kills about 140,000Americans and costs around $34 billion each year. About 87% of allstrokes result from ischemic strokes, in which blood flow to the brainis blocked by a mechanical clot (thrombus; FIG. 1A) or narrowing due toplaque buildup. The remaining 13% result from hemorrhagic strokes, whichoccur when a weakened blood vessel breaks, typically at an aneurysm(aneurysm; FIG. 1A), which is a localized bulge that has high risks ofrupture, causing bleeding into the brain. If not treated promptly, bothischemic and hemorrhagic strokes can lead to permanent brain damage,causing serious disorders with devastating effects on the quality oflife of the victims. When it comes to acute ischemic stroke, theimportance of early intervention is epitomized by the well-known phrase,“time is brain”, which emphasizes the fact that time is so critical whentreating a stroke to achieve better protection of the brain against theirreversible damage. In general, stroke victims who are treated within90 minutes have a substantially lower risk of permanent brain damage ordeath. The current treatment of acute ischemic stroke, however, stillrequires physically transporting patients to primary stroke centers,typically at large institutions such as university hospitals. Forpatients in distant or rural areas, where acute-care services are notavailable, the current systems of care have limitations inherent in thelogistics involved.

To enable early stroke intervention, it would be beneficial to enableremote surgery through teleoperated robotic systems. Such teleroboticsystems could potentially enable skilled neurosurgeons to performrequired surgical tasks remotely on patients at local hospitals,obviating the need to transport the patients to large institutions atthe expense of time. Despite the expected advantages, however, roboticapplications to cerebrovascular and endovascular neurosurgery arelacking. The challenge of realizing miniaturized robotic devices thatcan navigate through the narrow and complex cerebrovascular anatomy toreach to the target lesions in a minimally invasive manner has not beenmet with existing technologies. While several types of robotic cathetersand continuum robots have been commercialized so far, mostly for cardiacor pulmonary interventions, they are limited to large scales due tominiaturization challenges inherent in their conventional actuationmechanisms. As a result, even the most advanced form of continuum robotsdeveloped to date have remained substantially unsuited for neurosurgicalapplications due to considerably small and tortuous vascular structures.

In particular, in the current cerebrovascular and endovascularneurosurgery, manually controlled passive guidewires (FIGS. 1B-C), as agold standard, remain the only means to provide access to theintracranial vasculature in a noninvasive manner. For these guidewires,the distal tip of the device is typically pre-shaped with a fixedcurvature or shapeable into curved or bent shapes (FIG. 1C), instead ofbeing straight, for steering purpose. The pre-bent distal tip can beoriented towards a desired direction by manually twisting the proximalend of the device. After orienting the tip towards a desired directionthrough the twisting maneuver, the proximal end is pushed to advance thewhole device forward. Upon this pushing manipulation, the floppy tipconforms to the environment and passively follows the continuous path asthe guidewire moves forward (FIG. 1B). However, for this passivesteering method, typically multiple reshaping maneuvers are required toadjust the shape of the guidewire tip during the course of theprocedure. Furthermore, this twisting-based steering method oftenbecomes no longer effective, particularly when navigating through ahighly tortuous path due to the large friction acting on the devicealong the path. When forced to rotate further by additional twisting,the pre-shaped tip tends to jerk, abruptly rotating excessively in anunpredictable manner. This unwanted movement substantially compromisesthe controllability and maneuverability.

Several concepts of continuum robots have been developed, offering safertherapeutic and diagnostic procedures owing to the minimally invasivenature. Current robotic catheters with steering and navigationalcapabilities can be generally categorized into two types depending ontheir actuation mechanisms based on: (i) passive guidewires, which canbe manipulated, for example, by pulling/pushing antagonistic pairs ofmechanical wires (FIG. 2A), through a steerable sheath (e.g., Magellan™of Hansen Medical) driven by mechanical wires to steer a conventionalguidewire, which can be advanced/retracted or rotated by thelinear/rotary drive from the proximal end (FIGS. 2E-F), andadvancing/retracting or rotating a conventional passive guidewire with alinear/rotary drive from the proximal end (e.g., Corpath® GRX ofCorindus Vascular Robotics, FIG. 2H-I) or (ii) applying externalmagnetic fields to control rigid magnets embedded in the distal tip ofthe device (FIGS. 2B and 2J) and applying external magnetic fields tocontrol a finite-sized rigid magnet attached at the distal end tip of aguidewire, where the magnet is generally thicker than the guidewire(FIG. 2G). These devices are being used in clinical settings, mostly forcardiac (e.g. to treat arrhythmia) or pulmonary (e.g. for early diagnoseof lung cancer) applications by enabling minimally invasive access tothe targeted lesions through arteries or lung airways. However,application of such devices to cerebrovascular and endovascularneurosurgery have remained largely unexplored due mainly to the lack ofappropriate technologies. The biggest hurdle, thus far, is theminiaturization of such robotic devices to make them fit into thebrain's blood vessels, which are considerably smaller and more tortuousthan those in other body parts. For example, existing continuum robotsare limited to relatively large scale (>3 mm in diameter) due tominiaturization challenges inherent in their conventional actuationmechanisms. Tendon-driven continuum robots with antagonistic pairs ofwires are generally difficult to scale down to submillimeter size indiameter due to the increasing complexities in the fabrication processas the components become smaller. Magnetically steerable continuumrobots have also remained at large scales due to the finite size of theembedded magnets required to generate adequate deflection under appliedmagnetic fields. Additional limitations associated with the use of suchrigid magnets, particularly at submillimeter scale, are epitomized bythe fact that several products of magnet-tipped microguidewires seekingFDA premarket approval were later recalled because of the concern thatthe tiny magnets at the tip could break off, which may lead to undesiredclinical problems. The miniaturization challenges have rendered even themost advanced form of commercialized continuum robots, mostly used forcardiac and peripheral interventions, substantially unsuited forneurosurgical applications due to the considerably smaller and moretortuous vascular structures involved (FIGS. 2 C-D). Internal carotidarteries are already smaller than 5 mm in diameter, while cerebralarteries are typically smaller than 2.5 mm in diameter (FIG. 2D), whichmakes it almost impossible to use bulky robotic catheters.

Another challenge with current devices is posed by the use of X-ray forfluoroscopic imaging, which visualizes the state of the guidewire inreal time during the operation (FIG. 1D). Being exposed to continuousX-ray for a short period of time is generally considered innocuous forpatients. However, for neuroradiological interventionalists, cumulativeradiation exposure can pose a potential health threat, because theprotective gears cannot perfectly block the radiation.

Thus, further improvements in both devices and methods of use aregreatly needed.

SUMMARY OF THE INVENTION

The present invention provides a device for use in navigating throughhighly constrained environments, such as narrow and tortuousvasculature, based on active omnidirectional steering upon magneticactuation. In particular, the present invention provides asubmillimeter-scale ferromagnetic soft continuum robotic device havingan elongate body (generally in the form of a guidewire) composed of softpolymer matrices with dispersed hard-magnetic microparticles. An innercore is concentrically disposed within the soft polymer material alongat least a portion of the elongate body to provide further support andpushability of the device when navigating a desired pathway. This innercore can be in the form of a wire, which primarily provides support tothe soft polymer matrix material, or it can be configured to provide thesoft continuum robotic device with additional functionalities, e.g. byincorporating an optical fiber core for laser delivery.

According to one aspect, the present invention provides a continuumrobotic device for use in minimally invasive procedures comprising: anelongate body having a proximal end, a distal end, an inner core and anouter shell; the outer shell fabricated of an elastomeric material; aplurality of ferromagnetic particles dispersed within the outer shell;the elongate body having an initial shape. According to this aspect, theelongate body has an outer diameter of no greater than about 1000 μm,and exposure of the device to an external magnetic field magneticallyactivates the plurality of ferromagnetic particles to provide theelongate body in an activated shape different than the initial shape.

According to another aspect, the present invention provides a method ofperforming a minimally invasive procedures on the microvascular systemcomprising: providing a continuum robotic device comprising an elongatebody having a proximal end and a distal end, the elongate body includingan inner core and an outer shell; the outer shell fabricated of anelastomeric material; a plurality of ferromagnetic particles dispersedwithin the outer shell; and the elongate body having an initial shape;inserting the distal end into a blood vessel connected to one or moretarget sites of the microvascular system; and actively guiding thedistal end and advancing the elongate body through the microvascularsystem, including nonlinear branches of the microvascular system, to theone or more target sites using an external magnetic field to activatethe plurality of ferromagnetic particles, wherein the external magneticfield is selectively applied to the elongate body. According to thisembodiment, selectively exposing the elongate body to one or moreexternal magnetic fields is carried out so as to provide the elongatebody in a variety of activated shapes configured to guide the distal endand advance the elongate body through microvascular system to the one ormore target sites.

Embodiments according to these aspects may include one or more of thefollowing features. Exposure of the device to an external magnetic fieldmagnetically activates the plurality of ferromagnetic particles toprovide omnidirectional steering of the device. The plurality offerromagnetic particles are uniformly magnetized along an axialdirection of the elongate body. The plurality of ferromagnetic particlesare magnetized in nonuniform patterns of magnetic polarity along anaxial direction of the elongate body. The plurality of ferromagneticparticles are magnetized in patterns of magnetic polarity comprisingalternating patterns of different magnetic polarities. Alternatingpatterns of different magnetic polarities are disposed such thatexposure of the device to an external magnetic field magneticallyactivates the plurality of ferromagnetic particles to provide theelongate body in a wavy shape. The inner core and outer shell andfabricated, and the plurality of ferromagnetic particles are dispersed,such that exposure of the device to various direction and magnitudes ofexternal magnetic fields provides the body in a variety of activatedshapes different than the initial shape. The elongate body has an outerdiameter no greater than about 900 μm, more preferably no greater thanabout 850 μm, more preferably no greater than about 800 μm, morepreferably no greater than about 750 μm, more preferably no greater thanabout 700 μm, more preferably no greater than about 650 μm, and morepreferably no greater than about 600 μm. The device further comprises ahydrogel skin disposed on the outer shell. The hydrogel skin has athickness of about 10 μm to about 25 μm. The ferromagnetic particleshave an average particle size of about 2 μm to about 10 μm. The innercore is a metallic wire. The metallic wire is fabricated of superelasticnickel-titanium, stainless steel, platinum, a platinum-tungsten alloy, acobalt-chromium-molybdenum alloy, or combinations thereof. The innercore comprises one or more optical fibers. The elongate body furtherincludes one or more imaging, illumination, laser delivery, or sensingelements at or near the distal end. The inner core is a fiber opticshape sensor. The inner fore is a fiber optic shape sensor comprisingone or more Bragg grating. The outer shell is fabricated of one or morepolymer materials having a Young's modulus below 20 MPa. The outer shellis fabricated of an elastomer. The outer shell is fabricated of one ormore materials selected from natural rubbers, synthetic rubbers, andthermoplastic elastomers. The outer shell is fabricated of one or morematerials selected from silicone rubber, polyacrylate rubber,thermoplastic polyurethane, and styrene-ethylene-butylene-styrene(SEBS). The plurality of ferromagnetic particles are selected fromneodymium iron boron (NdFeB), samarium cobalt (SmCo),aluminum-nickel-cobalt (AlNiCo), copper-nickel-iron (CuNiFe),barium-iron oxide (BaFeO), platinum-cobalt alloys, and combinationsthereof. The ferromagnetic particles have programmed magnetic polaritiesthat enable magnetic actuation upon exposure to an external magneticfield. The plurality of ferromagnetic particles are coated with anon-corrosive layer. The non-corrosive layer is fabricated of one ormore materials selected from silica, Parylene C, gold, and epoxies. Theinner core extends through a portion of the elongate body that is lessthan an entire length of the elongate body. A portion of the elongatebody at the distal end does not include the inner core. The inner coretapers in diameter towards the distal end of the elongate body. Theelongate body comprises one or more magnetically active portionscomprising one or more outer shell portions containing a plurality offerromagnetic particles dispersed therein, and one or more inactiveportions comprising one or more one or more outer shell portions notcontaining a plurality of ferromagnetic particles dispersed therein. Adistal end portion of the elongate body comprises a distal magneticallyactive portion, and an adjacent portion of the elongate body proximalthe distal magnetically active portion comprises a magnetically inactiveportion. Selectively exposing the elongate body to one or more externalmagnetic fields creates multiple controllable modes and degrees ofbending of the elongate body depending on a direction and strength ofthe external magnetic field. A user performs the method remotely byviewing the device within the microvascular system using real timeimaging, and applying the one or more external magnetic fields using arobotic manipulation platform. The minimally invasive procedure is acerebrovascular or endovascular procedures. The one or more target sitesare selected from one or more aneurysms, embolisms, lesions, orarteries. The device inner core comprises one or more optical fiber, andthe device further includes a laser delivery element at the distal end,and the method further comprises guiding and advancing the distal end toone or more target sites selected from vascular occlusions,atherosclerosis, aneurysms, embolisms, and lesions, and treating the oneor more target sites with the laser. The device inner core comprises oneor more optical fiber, and the device further includes one or moreimaging, sensing, and/or illumination elements, and the method furthercomprises providing imaging, sensing, and/or illumination while guidingthe distal end and advancing the elongate body through the microvascularsystem to the one or more target sites. The device inner core comprisesone or more fiber optic shape sensors, and the method further comprisesusing the one or more fiber optic shape sensors to provide a user withreal-time feedback of a 3D shape of the elongate body. The one or morefiber optic shape sensors comprise one or more Bragg grating, and themethod further comprises exposing the one or more Bragg grating tostrain or temperature to shift a wavelength of the Bragg grating, anddetermining a direction and magnitude of the shift.

According to another aspect, the present invention provides a system forperforming a minimally invasive procedures on the microvascular systemcomprising a continuum robotic device for use in minimally invasiveprocedures comprising: an elongate body having a proximal end, a distalend, an inner core and an outer shell; the outer shell fabricated of anelastomeric material; a plurality of ferromagnetic particles dispersedwithin the outer shell; the elongate body having an initial shape;wherein the elongate body has an outer diameter of less than about 1000μm; and a control mechanism comprising a single permanent magnet heldand manipulated by a multi-degree of freedom (DOF) robotic arm, whereinexposure of the continuum robotic device to an external magnetic fieldfrom the control mechanism magnetically activates the plurality offerromagnetic particles to provide the elongate body in an activatedshape different than the initial shape.

Other systems, methods and features of the present invention will be orbecome apparent to one having ordinary skill in the art upon examiningthe following drawings and detailed description. It is intended that allsuch additional systems, methods, and features be included in thisdescription, be within the scope of the present invention and protectedby the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. The drawingsillustrate embodiments of the invention and, together with thedescription, serve to explain the principals of the invention.

FIGS. 1A-D schematically illustrate the challenges in currentcerebrovascular and endovascular neurosurgery, with FIG. 1A illustratingpathologic conditions in cerebrovascular arteries that can lead toeither ischemic (occlusion by a mechanical clot) or hemorrhagic(bleeding due to aneurysm rupture) stroke, FIG. 1B illustrating manuallycontrolled, passive guidewires used for providing minimally invasiveaccess to targeted lesions, FIG. 1C depicting two different types ofmanual guidewires with a pre-bent tip (left) and re-shapeable tip(right) for twisting-based, passive steering for navigating the narrowand highly nonlinear vascular structures, and FIG. 1D illustrating C-armfluoroscopy required for real-time imaging in endovascular neurosurgerybased on continuous X-ray, to which the surgeons are repeatedly exposedduring their careers.

FIGS. 2A-J schematically illustrate the challenges in existing continuumrobots and steerable robotic catheters, with FIG. 2A depictingtendon-driven continuum robots based on pulling/pushing antagonisticpairs of mechanical wires for applications in cardiac electrophysiology,FIG. 2B depicting magnetically steerable continuum robots having aseries of permanent magnets embedded in the distal tip of the device,FIG. 2C showing the considerably smaller and more tortuous vascularstructures involved in the cerebrovascular anatomy, FIG. 2D showing thatthe internal carotid arteries which are smaller than 5 mm in diameter,and the cerebral arteries which are typically smaller than 2.5 mm indiameter, FIGS. 2E-F showing a steerable sheath (Magellan™ of HansenMedical) driven by mechanical wires to steer a conventional guidewire,which can be advanced/retracted or rotated by the linear/rotary drivefrom the proximal end, FIG. 2G showing a magnetically controlledcatheter, with several rigid magnets embedded in the distal segment,used for cardiac electrophysiology to treat heart arrhythmia(Stereotaxis Inc.), FIGS. 2H-I showing a Corpath® GRX of CorindusVascular Robotics to advance/retract or rotate a conventional passiveguidewire with a linear/rotary drive from the proximal end, and FIG. 2Jshowing a magnet-tipped microguidewire with a finite-sized rigid magnetattached at the distal end tip, which is thicker than the guidewire.

FIGS. 3A-D schematically illustrate multifunctional soft continuumrobotic devices for use in cerebrovascular and endovascular procedures,with FIG. 3A illustrating the incorporation of a concentric functionalcore and a hydrogel skin coating on the outer surface of the core toprovide a self-lubricating layer to reduce the friction while navigatingthrough complex and constrained environments according to an embodimentof the present invention, FIG. 3B illustrating the incorporation of afunctional core in the form of an optical fiber to provide magneticallysteerable laser delivery or imaging for minimally invasive medicalapplications according to an embodiment of the present invention, FIG.3C illustrates navigation in the cerebrovasculature with a deviceaccording to an embodiment of the present invention based onmagnetically controlled active steering to enable access todifficult-to-reach environments such as cerebral aneurysms, and FIG. 3Dillustrates laser atherectomy using an embodiment of the presentinvention which incorporates a laser delivery into the core to removemechanical clots (thrombus) or plaque that cause obstructive vasculardiseases.

FIGS. 4B-D schematically illustrate a telerobotic manipulation platformfor magnetic actuation and control of the present invention deviceaccording to an embodiment of the present invention, with FIG. 4Adepicting a standard C-arm fluoroscope for real-time imaging of apatient's blood vessels based on continuous X-ray, FIG. 4B illustratinga 7-DOF (degrees of freedom) robotic arm holding a large cylindricalmagnet used to apply and control the actuating magnetic fields and fieldgradients for magnetic actuation and steering of the present inventiondevice based on a permanent magnet according to an embodiment of thepresent invention, where the magnet is aligned along a desired directionto induce the bending actuation on the soft continuum robotic device'smagnetically responsive tip along the desired direction, wherein thedegree of bending as determined by the applied field strength iscontrolled by adjusting the distance between the magnet and thesteerable tip by remotely controlling the robot arm with anyconventional controller mechanism (e.g. a joystick controller), withwhich an operator can advance/retract the guidewire and microcatheterunder visual feedback from real-time imaging, FIG. 4C illustrating therobot arm deployed and used in the operating room along with a C-armfluoroscope, FIG. 4D illustrating real-time teleoperation of the robotarm to control the position and orientation of the magnet with ajoystick controller, and FIG. 4E illustrating the magnetically steerablesoft continuum robotic guidewire steered by the magnet held by the robotarm to navigate through the neurovascular phantom.

FIGS. 5A-B schematically illustrate a magnetically steerable softcontinuum robotic device according to embodiments of the presentinvention, with FIG. 5A showing a magnetically responsive elongate bodywhich deflects along an externally applied magnetic field due to theincorporation of ferromagnetic microparticles with programmed magneticpolarities along the axial direction, and FIG. 5B illustrating aferromagnetic soft continuum robotic device according to an embodimentof the present invention with programmed magnetic polarities resultingfrom the hard-magnetic particles embedded in the elongate soft polymermatrix body, where a hydrogel skin provides a hydrated, self-lubricatinglayer on the device's surface, and where a silica shell coated aroundthe embedded magnetic particles prevents their corrosion at the hydratedinterface.

FIGS. 6A-C schematically illustrate a soft continuum robotic devicehaving an incorporated optical fiber as a functional core according toan embodiment of the present invention together with an experimentaldemonstration of the steerable laser delivery capability, with FIG. 6Billustrating the experimental setup for demonstrating steerable laserdelivery, and FIG. 6C illustrating close views of the laser-emitting tipwhich accurately points the small targets (2-mm dots) with the laserbeam in a prescribed order based on omnidirectional magnetic steering.

FIGS. 7A-D schematically illustrates fabrication methods for embodimentsof the present invention soft continuum robotic device, with FIG. 7Ashowing a conventional extrusion process commonly used for jacketingthermoplastic polymers around a core, FIG. 7B(i) showing anextrusion-based printing method for fabricating an elongate body withoutan inner core, FIG. 7B(ii) showing injection molding to incorporate aninner core by injecting a ferromagnetic composite ink into a mold whileplacing the concentric core inside the mold, FIG. 7C illustrating aferromagnetic composite ink based on PDMS+NdFeB (20 vol %) before andafter magnetization, and FIG. 7D illustrating hydrogel skin formationonto the outer surface of a ferromagnetic soft continuum robotic device.

FIGS. 8A-D schematically illustrate simulations for quantitativeprediction of multiple modes and degrees of bending of the softcontinuum robotic devices according to embodiments of the invention,wherein a distal end portion of the elongate body is composed with nocore, thus creating multiple modes and degrees of bending depending onthe direction and strength of the applied actuation field and theunconstrained length of the magnetically active segment, with FIG. 8Ashowing an unconstrained length equal to that of the “soft” segment,such that only the very distal end tip of the strongly reacts to theapplied magnetic fields, creating a J-shaped tip, FIGS. 8B-C showingthat as the unconstrained portion becomes longer, the bending stiffnessof the stiff segment decreases, increasing the radius of curvature ofoverall bending upon magnetic actuation, and FIG. 8D showing anexperimental demonstration of navigating through a highly nonlinear pathformed by a set of tightly spaced multiple rings, where the magneticfields for actuation (20 to 80 mT) were generated by a cylindricalpermanent magnet (diameter and height of 50 mm) at distance (from 40 to80 mm) where the proximal end was pushed to advance the magneticallysteered distal end of the device (having outer diameter 600 μm) duringthe navigation.

FIGS. 9A-B illustrate an experimental demonstration of a soft continuumrobotic device according to an embodiment of the present invention as itnavigates through a nonlinear path formed by a series of loosely spacedrings using active steering based on magnetic actuation.

FIGS. 10A-B schematically illustrate pathologic conditions inhard-to-reach areas across the human body (FIG. A), with FIG. 10Billustrating a soft continuum robotic device according to an embodimentof the present invention navigating through a complex neurovasculaturewith an aneurysm based on magnetic actuation.

FIG. 11 illustrates a demonstration of navigating through a 3Dneurovascular phantom network using a soft continuum robotic deviceaccording to an embodiment of the present invention, where the devicefirst passes through the sharp corner with acute angulation (between t=0s and t=5 s). The device makes another sharp turn after reaching thefirst aneurysm (t=11 s) based on the magnetic steering capability toreach the second aneurysm (t=15 s). Then, it makes another sharp turn atthe acute-angled corner beneath the second aneurysm (t=18 s) to reachthe third aneurysm (t=25 s), and navigates further downstream (t=36 s).

FIG. 12A-B illustrate a demonstration of steerable laser delivery inrealistic environments, where FIG. 12A shows a soft continuum roboticdevice emitting a laser beam at different target sites in a carotidartery phantom based on magnetic steering, and FIG. 12B shows navigationdownstream through the carotid artery after turning off the laser.

FIGS. 13A-F illustrate the rheological properties of ferromagneticcomposite inks, with FIG. 13A showing magnetization curves with magnetichysteresis loops of soft-magnetic and hard-magnetic materials, both ofwhich develop strong induced magnetization M when exposed to an externalmagnetizing field H_(c) with soft-magnetic materials forming a sharp andnarrow hysteresis curve due to the low coercivity (and hence do notsustain high remnant magnetization M_(r) independently of externalfields) and with hard-magnetic materials exhibiting much highercoercivity (retaining high remnant magnetization unless a strongdemagnetizing field beyond the coercivity is applied). FIG. 13Bgraphically illustrating the storage modulus plotted against the appliedshear stress for permanently magnetized composite inks with 10 vol %(bottom curve), 20 vol % (middle curve), and 30 vol % (top curve) ofNdFeB dispersed in uncured PDMS resin, where the crossover point atwhich the storage modulus becomes smaller than the loss modulus definesthe shear yield stress beyond which the magnetized composite paste canflow. The identified shear yield stresses for 20 vol % and 30 vol % inksare 165 kPa and 1640 kPa, respectively. The shear yield stress for 10vol % ink is smaller than the lower bound of the applied shear stress,and hence the data shows that the ink is already fluidized due to theapplied shear stress, and FIG. 13C graphically illustrating apparentviscosity plotted against the applied shear rate for 10 vol % (bottomcurve), 20 vol % (middle curve), and 30 vol % (top curve) inks. The datareveals shear-thinning behavior of the thixotropic paste of magnetizedferromagnetic composite ink, and FIGS. 13D-E illustrate the presence ofshear yield stresses in 20 vol % magnetized inks helps prevent theirphase separation due to gravitational sedimentation of the dispersedparticles over time where the vial on the left contains nonmagnetized,freely flowing composite ink, the middle vial contains alreadymagnetized composite ink, and the right vial contains nonmagnetized inkfirst loaded and then magnetized.

FIGS. 14A-F illustrate the characterizations of silica-coated magneticparticles, with FIG. 14A showing a schematic of the polycondensationreaction of TEOS in the presence of catalysts in basic conditions, wherethe nucleation and polymerization of TEOS lead to crosslinked layer ofsilica around the NdFeB particles, FIG. 14B shows a micro-computedtomography image of solidified ferromagnetic composite based on PDMS andsilica-coated NdFeB particles showing uniformly dispersed particles withno obvious gradient due to sedimentation, FIG. 14C shows a scanningelectron microscope image of silica-coated NdFeB particles indicatingthe size of a single particle, FIG. 14D shows a transmission electronmicroscope image of a silica coated NdFeB particle, from which thethickness of the silica layer is identified to be 10 nm, FIG. 14E showsFourier transform infrared spectroscopy of silica-coated NdFeB particlesclearly indicating the presence of Si—O—Si bonds, FIG. 14F shows theresults of leaching test of both uncoated and coated NdFeB particles in0.2 mM HCl solution (pH 3.7) for 3 days, where no visible change wasobserved in the silica-coated particles owing to the presence of theprotective silica layer, whereas the uncoated particles were highlyoxidized, turning the color of the solution yellow.

FIGS. 15A-J illustrate a hydrogel skin as a lubricating layer, with FIG.15A showing cross-sectional views of the coated specimen of PDFM+NdFeB(20 vol %) with hydrogel skin visualized by absorbed fluorescein, FIG.15B shows an uncoated specimen without hydrogel skin, where the dashedline indicates the boundary of the cross-section of the uncoatedspecimen, FIG. 15C shows top views of the coated specimen with hydrogelskin, FIG. 15D shows the same top view of an uncoated specimen, wherethe fluorescing specks visible in the uncoated sample are due toresidual fluorescein adsorbed onto the surface, FIG. 15E shows aschematic of testing setup for measuring friction coefficients using arheometer, FIG. 15F shows a schematic of testing setup for measuringforce required to pull a cylindrical specimen (diameter of 8 mm) at aconstant speed under applied normal force by the pair of grips, FIG. 15Gshows a semi-log plot of the pulling force measured over time during thepullout test performed at 200 mm/min for both coated and uncoatedspecimens under two different normal force conditions (2 N and 5 N),FIG. 15H shows friction coefficients measured from both coated anduncoated samples under different shear rates and FIG. 15I under normalpressure, FIG. 15J showing friction coefficients measured from prolongedshearing of both coated and uncoated samples up to 60 min at shear rateof 0.5 s⁻¹ under normal pressure of 6 kPa. The error bars in FIGS. 15H-Jindicate the standard deviations of the mean values obtained from 5different measurements.

FIGS. 16A-I illustrate an example of optimizing the design of theferromagnetic soft continuum robotic device according to embodiments ofthe present invention, with FIG. 16A showing a schematic of the devicewith uniform magnetization M along the axial direction deflectingtowards the direction of the uniform magnetic field B appliedperpendicularly to the body (the unconstrained length and the outerdiameter of the robot are denoted L and D, respectively. δ indicates thedeflection of the free end, and θ indicates the deflection angle), FIG.16B graphically illustrates the magnitude of magnetization (denoted M)linearly varying with the volume fraction of the embedded magneticparticles, FIG. 16C graphically illustrates the shear modulus (denotedG) of the ferromagnetic composite at different particle concentrations,FIG. 16D graphically illustrates a prediction of the variation of M/G, acharacteristic quantity that determines the degree of deflection forsmall bending, with the particle volume fraction under given appliedfield strength for a given geometry (the unit of this quantity, T⁻¹, orequivalently Am/N, is intentionally omitted for simplicity), FIG. 16Egraphically illustrates the actuation angle predicted from finiteelement simulation and experimental measurements plotted against theapplied field strength normalized by material properties (M and G) for aparticular composition (20 vol %) with different aspect ratios: L/D=10,15, 20, FIG. 16F graphically illustrates the variation of actuationangle with particle concentration at different actuation fieldstrengths: B=10, 20, 40, 80 mT, predicted from simulation results whenL/D=10, FIG. 16G graphically illustrates predictions of the variation ofM²/G, a quantity that characterizes the energy density in a deflectedbody for small bending case, with the particle volume fraction undergiven applied field strength for a given geometry. The unit of thisquantity, A²/N, is intentionally omitted for brevity, and FIGS. 16H-Igraphically depict the average energy density predicted by finiteelement simulations for small (FIG. 16H) and large (FIG. 16I) bendingcases, as a function of particle concentration.

FIGS. 17A-B show experimental setups and dimensions for thedemonstrations of active steering and navigating capabilities offerromagnetic soft continuum robotic devices according to embodiments ofthe present invention, based on magnetic actuation, where FIG. 17A showsa set of rings loosely placed at different locations (in mm) withdifferent heights (in mm) and tilt angles, through which the devicesnavigate selectively based on the demonstrated active steeringcapabilities, and FIG. 17B shows set of rings more tightly placed toform a tortuous path.

FIGS. 18A-B show a magnetic actuation and steering mechanism based on acylindrical permanent magnet, where FIG. 18A shows a schematic of acylindrical magnet of radius R and height H and the magnetic fieldaround the magnet, and FIG. 18B schematically depicts a basic principlefor magnetic actuation and steering of the present inventionferromagnetic soft continuum robotic devices employed in theexperimental demonstrations herein, where the central axis of the magnetis aligned along the desired direction to induce the bending actuationon the device's magnetically responsive tip along the desired direction.The degree of bending, which is determined by the applied fieldstrength, is controlled by adjusting the distance between the magnet andthe robot.

FIG. 19 shows influence of spatial gradients of the actuating magneticfield on the actuation and steering of the ferromagnetic soft continuumrobot according to an embodiment of the present invention, where themagnetic field B generated along the central axis of a large cylindricalmagnet is applied perpendicularly to the magnetization vector M alongthe body, which gives rise to magnetic body torques (τ^(magnetic)) thatdrive the bending actuation. As the body deforms under the applied fieldwith spatial gradients along the field direction, magnetic body forces(b^(magnetic)) are generated. As the magnetization in the currentconfiguration FM becomes more aligned with the applied field direction,the magnetic body force increases whereas the magnetic body torquedecreases. This means that, when the robotic device is actuated andcontrolled with a magnet, the bending actuation is initiated and drivenby the magnetic body torque and then further supported by the magneticbody force as the robot's elongate body deforms.

FIG. 20A-C depict dimensions and details of the cerebrovascular phantommodel used for the experimental demonstration of the active steering andnavigating capabilities of the present invention ferromagnetic softcontinuum robotic device, where FIG. 20A is a detailed view and relevantdimensions of the vascular structure of the phantom model around aparticular cerebrovascular anatomy (the circle of Willis), as well asthe surrounding arteries with multiple aneurysms. The path that isnavigated, from the carotid artery to the right middle cerebral artery,is indicated with dashed lines, FIG. 20B illustrates the vascularanatomy and dimensions of the real-sized phantom model (top view), andFIG. 20C illustrates the vascular structure and dimensions of thephantom model (side view) around the path.

DETAILED DESCRIPTION

The present invention generally provides a device and method for use inminimally invasive procedures, particularly procedures on the vascularsystem including cerebrovascular and endovascular neurosurgicalprocedures. More particularly, the present invention provides asubmillimeter-scale soft continuum robotic device adapted for navigatingthe complex vascular system, including the microvascular system. Thedevice is provided with omnidirectional steering and navigatingcapabilities based on magnetic actuation, which are enabled byprogrammed magnetic polarities in the soft body of the device. An outersurface of the device may be provided with a material, such as ahydrogel skin, to minimize friction as the device navigates throughcomplex and constrained environments, such as a tortuous cerebrovascularsystem with multiple aneurysms.

The present invention small-scale soft continuum robotic device, whichis provided with self-contained actuation and intuitive manipulation,enables access to hard-to-reach areas and, thus, provides numerousbenefits in diverse areas, particularly in medical applications such asminimally invasive surgery.

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

According to an embodiment of the present invention, a soft continuumrobotic device 1 is generally provided with an elongate body 2 in thegeneral form of a guidewire. The elongate body 2 includes a core-shelltype structure in which an inner core 3 is disposed within andsurrounded by an outer shell 4. Dispersed within the outer shell 4 areferromagnetic particles 6 as distributed actuation sources to enablenavigation and control of the soft continuum robotic device 1 throughthe application of an external magnetic field.

Because the present invention is directed towards and used in minimallyinvasive procedures (e.g., targeting lesions in intracranial arteries),particularly wherein microvascular structures must be navigated, theelongate body 2 is suitably provided with a diameter that is suitablysized to navigate the small and non-linear pathways, and is fabricatedof materials that provide the required bendability and manipulation ofthe elongate body 2 along its length. As such, according to preferredembodiments of the invention, the elongate body 2 has an overalldiameter no greater than about 2500 μm, more preferably no greater thanabout 2000 μm, more preferably no greater than about 1500 μm, morepreferably no greater than about 1000 μm, more preferably no greaterthan about 950 μm, more preferably no greater than about 900 μm, morepreferably no greater than about 850 μm, 800 μm, more preferably nogreater than about 750 μm, more preferably no greater than about 700 μm,more preferably no greater than about 650 μm, and more preferably nogreater than about 600 μm. According to an exemplary embodiment, theoverall diameter of the elongate body 2 ranges from about 250-1000 μm,and in some embodiments, from about 250-600 μm. Due to the core-shellstructure of the guidewire, the overall diameter is at least as large asthe internal core 3 required to provide adequate structure to theelongate body 2, plus the outer shell 4 layer which accommodates theferromagnetic particles 6. Typically, internal cores of guidewires rangein diameter from about 80-400 μm. In embodiments where typical internalguidewire core sizes are used for the inner core 3, the outer shell 4will generally make up the remainder of the total overall diameter. Asnoted, in some embodiments a hydrogel skin is provided on the outermostsurface of the elongate body 2. Such hydrogel skin layers are generallythin (e.g. 10-25 μm) layers. As such, in certain embodiments, where theabove-noted ranges for the overall diameter and inner core 3 apply, theouter shell 4 plus the hydrogel skin will generally make up theremainder of the total overall diameter.

As illustrated, the inner core 3 is disposed within and is entirelysurrounded by the outer shell 4 (e.g. see FIGS. 3A-B, 6A, 7B). The innercore 3 is equipped with additional mechanical support and/orfunctionalities, to provide a functional core structure. For example, inorder to provide mechanical support and pushability required fornavigating the elongate body 2 through complex and narrow pathways, theinner core 3 is generally in the form of a wire. According to variousembodiments, the inner core 3 is in the form of a thin metallic wire. Onpreferred example of a suitable thin metallic wire is a superelasticnickel-titanium (nitinol) wire, or other metallic alloys. However, theinner core 3 may be suitably be fabricated of any other materialsconventionally used in forming the wire portions of conventionalguidewires, including but not limited to stainless steel, platinum,platinum-tungsten alloy, cobalt-chromium-molybdenum alloy, Scitanium®,MP35N® alloys, etc.

If it is desired to provide further functionalities to the softcontinuum robotic device 1, a variety of functional inner cores 3 may beincorporated within the outer shell 4. For example, the inner core 3 maybe formed of one or a bundle of optical fibers 5 to enable additionalfunctions such as imaging, illumination, or laser delivery (e.g., seeFIGS. 3B, 6A). As such, the present invention soft continuum roboticdevice 1 will be capable of delivering a laser beam through the embeddedoptical fiber along its elongate body 2 (FIGS. 3B, 6A). It is noted thatsince the transmission of light through the incorporated optical fibers5 is not affected by the externally applied magnetic fields foractuation, the soft continuum robotic device 1 can still be magneticallyactuated and steered to enable the navigational tasks. Potentialapplications of such multifunctional soft continuum robots includelaser-assisted treatment of vascular occlusion (blockage due to athrombus) or atherosclerosis (narrowing due to plaque buildup),so-called laser atherectomy (e.g., FIG. 3D). The ability to keep thelaser-emitting tip of the soft continuum robotic device 1 in positionbased on magnetic steering will minimize unwanted motion or displacementof the tip from the desired location during the laser ablation, therebyimproving the accuracy and the safety of such procedures.

Based on the omnidirectional steering capability, a soft continuumrobotic device 1 equipped with an optical fiber inner core 3 was used todemonstrate magnetically steerable laser delivery (see FIGS. 6A-C). Asdepicted, the soft continuum robotic device 1 was manipulated usingmagnetic actuation so as to accurately point the laser beam at the smalltargets in a desired order.

According to some embodiments, an optical fiber inner core 3 is providedin the form of a fiber optic shape sensor configured to facilitatemonitoring and tracking the shape of the elongate body 2. The presentinvention inner core 3 in the form of a fiber optic shape sensor can bestructured and configured in accordance with any conventional fiberoptic shape sensors. Incorporating this shape-sensing capability intothe present invention soft continuum robotic device 1 can provide a userwith an improved ability to more accurately navigate and control thedevice using an external magnetic field. In particular, providing theinner core 3 in the form of a fiber optic shape sensor enables real-timefeedback of the 3D shape of the elongate body 2 for better tracking,steering, and navigational purposes.

According to an exemplary embodiment, the fiber optic shape sensor is inthe form of a Fiber Bragg Grating (FBG) array, which is generally amicrostructure that is patterned in the core of a fiber optic. Anynumber of FBG's may be patterned along the length of a fiber optic, witheach FBG providing an invisible reflector disposed inside the core ofthe optical fiber with specific working wavelength. FBG arrays are knownin the art and, thus, the present invention fiber optic shape sensor maybe in accordance with any of these known FBG array structures andconfigurations. Upon exposure of the FBG array to strain or temperature,the Bragg wavelength of the FBG shifts (either increases or decreases),where the direction and magnitude are determined by the change in theapplied strain or temperature.

The shape-sensing capability provided by the present invention will,thus, enable (1) using a full 3D roadmap (instead of the current 2Droadmaps) in various procedures (including endovascular and endoscopicprocedures) while minimizing radiation exposure due to the reduced useof fluoroscopy based on continuous X-ray and (2) more precise andaccurate control of continuum robotic devices as a consequence of theability to implement feedback control, which is enabled by thestate-awareness through the embedded shape sensors.

In addition to laser delivery functionalities, the inner core 3 can besuitably designed to provide a number of other applications. Forinstance, when equipped with a miniature CMOS sensor, while havingmultiple functional cores for both illumination and imaging, the softcontinuum robotic device 1 may further enable submillimeter-scaleendoscopic procedures such as angioscopy to better diagnose pathologicconditions such as embolism or aneurysms in the affected sites ofintracranial arteries. Such sites are far less accessible due to theconsiderably small and tortuous vascular structures.

As set out, the elongate body 2 of the soft continuum robotic device 1includes an outer shell 4. Generally, the outer shell material 4 can beselected from any materials that provide the necessary maneuverabilityand flexibility to the elongate body 2 so that the elongate body cannavigate narrow and complex vascular pathways discussed herein.According to embodiments of the present invention, the outer shell 4 isfabricated of one or more soft polymer matrix materials. In principle,any elastomer (soft polymers with low Young's modulus; e.g. below 20MPa) could suitably be used, and is limited only by the upper stiffnessvalue at which adequate bending and manipulation of the elongate body 2would not be possible upon magnetic actuation. For example, somesuitable materials for use as the outer shell 4 material include, butare not limited to, natural/synthetic rubbers (e.g. silicone rubber,polyacrylate rubber, etc., which would be crosslinked (vulcanized)) andthermoplastic elastomers (thermoplastic polyurethane,styrene-ethylene-butylene-styrene (SEBS), etc., which aremelt-processible and do not require crosslinking). According to someembodiments, the outer shell 4 could be formed using commercialguidewire fabrication methods, e.g., by using a typical jacketingprocess based on extruding polymer melts (e.g. TPU) around a core wire(inner core 3). Because the soft continuum robotic device 1 is used byinserting the elongate body 2 within the vasculature of a patient, theall materials used in forming the soft continuum robotic device 1, andespecially the outer shell 4 are biocompatible.

In order to provide the soft continuum robotic device 1 with remotecontrol capabilities via magnetic fields, a plurality of magnetizableferromagnetic particles 6 (preferably microparticles) are dispersedwithin the outer shell 4 material (e.g., see FIGS. 3A, 5A-B). Someexamples of suitable magnetizable ferromagnetic particles 6 include, butare not limited to, neodymium iron boron (NdFeB), samarium cobalt(SmCo), aluminum-nickel-cobalt (AlNiCo), copper-nickel-iron (CuNiFe),barium-iron oxide (BaFeO), platinum-cobalt alloys, and combinationsthereof. The magnetizable ferromagnetic particles 6 are provided withmagnetic polarities that enable magnetic actuation when external fieldsare applied (e.g., see FIGS. 5A-B)

Ferromagnetic alloys, in general, have highly corrosive nature due tothe high content of iron. In embodiments which include a hydrogel skinon an outer surface of the elongate body 2, in order to preventcorrosion of the embedded ferromagnetic particles (e.g., NdFeB) at thehydrated interface with the water-containing hydrogel skin, theferromagnetic particles are coated with a thin shell of non-corrosivematerial. Any conventional non-corrosive material may be suitably beused and may include, for example, silica (glass). Parylene C, as wellas a variety of non-corrosive and non-magnetic metals (e.g., gold etc),as well as epoxies. One example of a non-corrosive material is silica(FIG. 5C) based on the condensation reaction of tetraethylorthosilicate(TEOS), which nucleates around the particles to form a crosslinkedsilica layer (FIG. 14A). The resulting silica shell was identified to beabout 10-nm thick from transmission electron microscope (TEM) imaging(FIG. 14D) and as further verified by Fourier transform infraredspectroscopy, which clearly indicates the presence of Si—O—Si bonds(FIG. 14E). The effectiveness of the silica shell in preventing thecorrosion of NdFeB particles was verified by performing a leaching testfor both coated and uncoated particles with a weak acidic solution (0.2mM HCl; pH 3). The results showed highly oxidized uncoated particles butno visible change in silica-coated particles, which illustrated theanti-corrosion effect of the silica shell formed around the NdFeBparticles (FIG. 14F). Due to the marginal thickness of the silica shellthat was formed as compared to the size of microparticles, the silicacoating resulted in a slight increase in volume, which was roughlyestimated to be around 1% when assuming a uniform silica layer around aspherical ferromagnetic particle.

By incorporating the ferromagnetic particles 6 into the outer shell 4,the present invention soft continuum robotic device 1 is provided withactive, omnidirectional steering upon magnetic actuation, which is basedon the deformation of the magnetically active elongate body 2 inresponse to the externally applied magnetic field. Using this magneticsteerability, the soft continuum robotic device 1 can be selectivelynavigated through desired paths.

According to various embodiments, in order to further reduce frictiongenerated when the soft continuum robotic device 1 navigates throughcomplex and constrained environments, the surface of the elongate body 2can be coated with a thin, lubricious layer of hydrophilic polymers(e.g., see FIGS. 3A, 5B). Such hydrophilic coatings may be in accordancewith conventional hydrophilic coatings provided on medical devices, andcould further be applied using any convention hydrophilic coatingtechnique. For example, according to an exemplary embodiment, a hydrogelskin may be grown to provide a thin (e.g. 10-25 μm) layer of hydratedcrosslinked polymers onto the surface of the elongate body 2. Thishydrogel skin substantially decreases the surface friction (e.g., bymore than 10 times) due to its high water content.

According to an exemplary embodiment, the hydrogel skin is formed of acrosslinked network of hydrophilic polymers (polydimethylacrylamide;PDMAA) that are grafted onto the elastomer chains on the elongate body 2surface. For the hydrogel coating procedure, the following protocol maybe followed: First, the solidified elongate body 2 is treated with anorganic solution based on ethyl alcohol that contains hydrophobicphotoinitiators (benzophenone). Exposure to this organic solutioninduces swelling-driven absorption of the photoinitiators into theelongate body 2 surface. The treated elongate body 2 is then immersedinto a hydrogel monomer (DMAA) solution (FIG. 7D) containing hydrophilicphotoinitiators (Irgacure-2959). Upon exposure to ultraviolet (UV)radiation (FIG. 7D), the hydrogel monomers are polymerized by thehydrophilic initiators while at the same time covalently grafted ontothe surface-bound elastomers by the activated benzophenone, leaving athin hydrogel-polymer interpenetrated layer on the surface. Thethickness of the hydrogel skin is measured to be 10-25 μm fromfluorescence microscope images taken from coated and uncoated sampleswith planar geometry (1-mm-thick sheet) (FIG. 15A-D). The microscopicimages clearly identify the presence of the hydrogel skin on the coatedsamples.

The resulting hydrogel skin was shown to dramatically reduce the surfacefriction, which is characterized by the friction coefficients measuredfrom a rheometer testing while applying different levels of shear ratesand normal pressure (FIG. 15E). The measurements show a tenfold decreasein the friction coefficient (FIGS. 15H-I) as a result of the lubricioushydrogel skin in all given conditions. Furthermore, the coated hydrogelskin was demonstrated to remain stable and undamaged even afterprolonged shearing over an hour, exhibiting sufficient mechanicalrobustness (FIG. 15J). Forces required to pull cylindrical specimenswith and without hydrogel skins were also experimentally measured at aconstant speed (200 mm/min) under different normal forces (2 N and 5 N)applied by a pair of grips (FIG. 15F). The results show substantialdecrease in the pulling force as a consequence of the self-lubricatinghydrogel skin (FIG. 15G). When the applied normal force was 2 N, thehydrogel skin reduced the pulling force by a factor of 15 (from 2.65 Nto 0.18 N). As the normal force was increased from 2 N to 5 N, the forcerequired to pull the same uncoated specimen at the same rate increasedby 150%. Compared to this, the required force to pull the coatedspecimen increased by only 60%, which illustrates how effectively theself-lubricating hydrogel skin is capable of reducing the surfacefriction under the increased load.

According to various embodiments, in addition to providing an inner core3 for overall enhanced pushability and support of the elongate body 2,one or more selective portion(s) of the elongate body 2 havingferromagnetic particles 6 incorporated therein may be fabricated so asto not include an inner core 3. In other words, for example, where it isdesired to have a distal end portion 7 that is softer and moreflexible/more manipulatable than the remainder of the elongate body 2having the inner core 3 disposed thereon, the inner core 3 may bedesigned to extend up to but not through the distal end portion 7. Assuch, in this embodiment, the selected distal end portion 7 is composedof the ferromagnetic soft polymer composite only (i.e., it excludes theinner core 3) and, thus, is substantially softer and more flexible/moremanipulatable than the remainder of the elongate body 2 which containsthe inner core 3. The softer and hence more responsive distal endportion 7 enables creating multiple modes and degrees of bendingdepending on the direction and strength of the applied actuation fieldand the unconstrained (i.e., coreless) length of the magnetically activedistal end portion 7.

This is demonstrated in the simulation results presented in FIGS. 8A-D,when the unconstrained length of the stiff segment equals that of thesoft segment, only the very end tip of the continuum robot stronglyreacts to the applied magnetic fields, creating a J-shaped tip (FIG.8A). This is because the short unconstrained segment has large bendingstiffness. As the unconstrained part becomes longer, the bendingstiffness of the stiff segment decreases, increasing the radius ofcurvature of overall bending upon magnetic actuation (FIGS. 8A-C). Thismultiple modes capability and degrees of bending enables the softcontinuum robotic device 1 to make sharp turns and hence navigatethrough a highly nonlinear, tortuous path, as demonstrated in FIG. 8Dwhere the tortuous path is created by a series of closely spaced rings.

In addition, by providing a distal end portion 7 that is substantiallysofter and more flexible than the remainder of the elongate body 2, thedevice will further minimize any damage, such as puncture or rupture, tointernal structures that it comes into contact with (e.g., internalwalls of blood vessels).

Suitable fabrication methods for the present invention soft continuumrobotic device 1 may be chosen depending on the type of material (e.g.,type of soft polymer matrix) used in forming the outer shell 4 of theelongate body 2.

According to various embodiments, conventional extrusion processes,which are used in forming commercial guidewires having polymer jacketsand core wires, could suitably be used. In this conventional process,thermoplastic polymers (e.g. TPU) are heated and mixed with metallicpowder (e.g. tungsten; for radiopacity to make the body visible underX-ray). This polymer melt mixture is then extruded around a core wireinto a water bath for immediate cooling. Upon cooling, the TPU quicklysolidifies. In order to form the present invention device 1, theselected outer shell 3 material is provided and mixed with the magneticparticles 6 at the desired concentration(s).

Further exemplary fabrication methods are depicted in FIG. 7. Forexample, a conventional extrusion processes (FIG. 7A(i)) used forguidewire jacketing can be suitably used for forming an elongate body 2(particularly the outer shell 4 portion) out of thermoplastic polymerswhile forming the concentric inner core 3 therein. Other extrusion-basedprocesses, such as 3D printing (FIG. 7B(i)) can be suitably used forforming an elongate body 2 (particularly the outer shell portion 4) outof both thermoplastic polymers as well as silicone-based ferromagneticsoft composite materials.

According to an exemplary embodiment, in a 3D printing process, uncuredferromagnetic composite ink consisting of silicone elastomer resin mixedwith ferromagnetic particles 6 (or other suitable soft polymer matrixmaterial with incorporated ferromagnetic particles 6) is used to form anoverall elongate body member 2 (FIG. 7B(i)). As depicted, the formedstructure does not incorporate the inner core 3. To fabricate anelongate body which incorporates a functional inner core 3, FIG. 7B(ii)shows an injection molding process in which a micro-mold in acylindrical tube shape can be used to form the inner core 3. Inparticular, the ferromagnetic composite ink forming the outer shell 4 isinjected while placing the concentric core inside the mold.

More specifically, according to various embodiments, the elongate body 2may include one or more magnetically responsive portions, such as distalend portion, followed by a magnetically inactive segment (FIGS. 8A-C,9A). The device can be fabricated as such by either printing orinjection molding, both of which require extruding the thixotropicpaste-like ink through a micro-nozzle by applying pressure (FIG. 7B).The printing technique differs from conventional extrusion of moltenthermoplastic polymers in a sense that it does not require any heatingto melt and fluidize the ink. The shear-thinning behavior of themagnetized ink ensures that the composite ink can be easily extrudedwhen pressurized, while the presence of yield stress helps the depositedink maintain its shape instead of spreading and becoming flat (FIG. 7C).To provide additional mechanical support or functionalities in theelongate body 2, an inner core 3 is incorporated into the elongate body(at the desired locations) through injection molding. For this process,a micro-tube is used as a mold, into which the thixotropic composite inkis injected while locating a concentric functional core inside the mold.Once the printing or injection is complete, the printed or molded inkundergoes thermal curing (PDMS-based composite) or solvent evaporation(TPU-based composite) upon heating to solidify into the elongate body 2.During the heating process, the presence of yield stress may help theunsolidified ink maintain its shape on the printing substrate or remainstable in the mold instead of flowing and escaping due to the decreasein viscosity at the elevated temperature. Thereafter, the magneticallyactive portion(s) are uniformly magnetized again, along the axialdirection to possess programmed magnetic polarities required to createdeflection upon magnetic actuation (FIGS. 5A-B).

According to various embodiments, more complex patterns of magneticpolarities, other than uniform magnetization along the axial direction,can also be programmed depending on the functional requirements of thesoft continuum robotic device 1. As an example, an alternating patternof magnetic polarities can be programmed to create wavy shape underapplied magnetic fields. Such nonuniform magnetization pattern can berealized by several different methods. First, while printing theferromagnetic composite ink to fabricate the elongate body 2 outer shellportion 4, magnetic fields can be applied around the printing nozzle tomake the embedded ferromagnetic particles 6 reorient along the appliedfield directions. Second, after the fabrication of the elongate body 2is complete, the elongate body 2 can be first deformed into a certaindesired shape, and then a strong impulse magnetic field can be appliedto permanently magnetize (or magnetically saturate) the embeddedferromagnetic particles 6 to achieve a desired nonuniform magnetizationprofile.

Ferromagnetic materials in general develop strong induced magnetizationunder applied magnetic fields. Unlike soft-magnetic materials, such aspure iron, which easily lose the induced magnetization once the externalfield is removed, hard-magnetic materials, such as neodymium-iron-boron(NdFeB), are characterized by their ability to retain high remnantmagnetization against the external field once they are magneticallysaturated due to their high coercivity (FIG. 13A). The main body of thesoft continuum robotic device 1 is made of an elastomer composite thatcontains magnetizable microparticles (5-μm-sized on average; FIG. 14C)of NdFeB. The soft polymer matrix of the robot's body is composed ofeither silicone (polydimethylsiloxane; PDMS) or thermoplasticpolyurethane (TPU) elastomers, depending on desired mechanicalproperties.

As the initial step of the fabrication process, the ferromagneticcomposite ink is prepared by homogeneously mixing nonmagnetized NdFeBparticles with an average size of 5 (or other desired ferromagneticparticles) with uncured PDMS resin or TPU dissolved (5 wt %) in solvent(N,N-dimethylformamide) (or other suitable soft polymeric matrixmaterial) at a desired volume fraction (e.g using a planetary mixer suchas AR-100, Thinky, at 2,000 r.p.m. for 2 min). For PDMS based ink, 5 wt% of curing agent containing platinum catalyst was added in thesubsequent mixing for 45 sec under the same condition, after coolingdown at room temperature for 1 min. To impart desired rheologicalproperties to the mixture for ease of fabrication later, the wholemixture was magnetized by applying a strong impulse of magnetic fieldsto magnetically saturate the dispersed NdFeB particles. For example,using impulse magnetic fields (about 2.7 T) generated by an impulsemagnetizer (IM-10-30, ASC Scientific).

This turns the previously freely flowing mixture into a thixotropicpaste (FIG. 7C) with shear-yielding (FIG. 13B) and shear-thinning (FIG.13C) properties due to the strong interaction between the permanentlymagnetized NdFeB microparticles. The acquired rheological propertiesafter magnetization are important fabrication, as detailed in thefollowing section, but also conducive to preventing phase separation ofthe composite ink due to sedimentation of the dispersed particles overtime (FIGS. 13D-F). The suppressed phase separation providesmicrostructural uniformity (FIG. 14B), which assumes formation of ahomogeneous continuum when modeling the macroscopic behavior of thematerial to quantitatively predict the response of the present inventionsoft continuum robotic device 1 upon magnetic actuation.

According to embodiments of the present invention, a control mechanismin the form of a robotic manipulation platform may be used to generatemagnetic fields required to actuate and control the soft continuumrobotic device 1.

According to an embodiment of the present invention, the soft continuumrobotic device 1 is controlled through a control mechanism in the formof a single permanent magnet held and manipulated by a multi-degree offreedom (DOF) robotic arm. For example, a 7-DOF robotic arm holing alarge permanent magnet can be configured to control the steering amountand direction by varying the applied field strength and direction. Amotorized device to precisely insert and advance the soft continuumrobotic device 1, from its proximal end upon remote control at theworkstation console, may also be provided to enable teleoperation. Oneexemplary embodiment of this type of control mechanism is depicted inFIGS. 4B-C. The control mechanism is both compact and efficientmechanisms of generating and control magnetic fields for actuating themagnetically steerable soft continuum robotic device 1. In particular,the single permanent magnet and robotic arm are configured and arrangedsuch that changing the orientation and distance of the magnet relativeto the soft continuum robotic device 1, results in modifications to thedirection and strength of the applied magnetic field. Thus, a humanoperator can remotely control (e.g., teleoperate) the robotic arm.According to embodiments of the invention, standard fluoroscopicimaging, which visualizes radiopaque components or markers (such as goldor tungsten) of guidewires/catheters, will be directly applicable byincorporating in the soft continuum robotic device 1 a sufficient volumeof radiopaque magnetic particles that are visible under X-ray. In theseembodiments, the present system can be integrated with a standard C-armfluoroscope (e.g., FIG. 4C) or other suitable mechanism to enableobservation of the state of the soft continuum robotic device so that asurgeon can remotely manipulate the device while receiving real-timevisual feedback in the control of the robotic arm.

According to alternate embodiments, commercialized magnetic manipulationsystems based on (i) a pair of large magnets controlled by robotic armssuch as Stereotaxis Niobe® or Genesis' or (ii) multiaxialelectromagnetic coils such as Magnetecs CGCI or Aeon Phocus may beadopted for use with the present invention soft continuum robotic device1.

According to an embodiment of the present invention, the capability ofthe present invention ferromagnetic soft continuum robotic device 1 tonavigate complex and constrained environments is demonstrated. Suchnavigation is based upon active steering using magnetic actuation incombination with the ferromagnetic particles 6 dispersed within theouter shell 4. In addition, further functionalities (laser delivery,imaging, illumination, CMOS sensing, etc) may be enabled byincorporating a functional inner core 3 (e.g., one or a bundle ofoptical fibers)

FIGS. 9A-B illustrate an embodiment of the device passing through a setof rings using the magnetically responsive distal end portion 7, whichfollows the direction in which the actuating field is applied. Forexperimental demonstration, a cylindrical permanent magnet (diameter andheight of 50 mm) was employed to apply the actuating magnetic fields atdistance. The basic principle for magnetic actuation and steering is toalign the central axis (denoted z-axis in FIG. 18A) of the magnet alongthe desired direction to induce bending of the device distal end portion7 towards the desired direction (FIG. 18B). Although the bendingactuation in general is driven by magnetic body torques as discussedearlier, the spatial gradients of applied magnetic fields can also giverise to magnetic body forces, which further encourage the device'sdistal end portion 7 to align itself along the magnet's central axis(FIG. 19), as discussed in the Supplementary Text at the end of thedisclosure.

FIG. 9B shows an experimental demonstration of the fabricated prototype,which selectively navigates through a set of loosely placed rings (seeFIG. 17A for details) based on magnetic actuation and steering achievedby manually manipulating a single magnet. The demonstrated prototype wasfabricated through injection molding (FIG. 7B) of a PDMS+NdFeB compositeink being 600 μm in diameter. To provide mechanical support andpushability, a nickel-titanium alloy (nitinol) core was incorporated(FIG. 9A) in the robot's body. Since the nitinol core was from the tipof a commercial guidewire, the magnetically responsive tip is naturallyconnected to the commercial guidewire (see Materials and Methods belowfor details). The navigating performance of the prototypes showed anaverage time taken to complete the demonstrated task: 50±1.58 sec.

To enable making sharp turns and hence navigating through a tortuouspath, a variation in the bending stiffness of the magneticallyresponsive portion(s) of the soft continuum robotic device 1 wasintroduced. The resulting continuum robotic device 1 (diameter of 600μm) had a short (3-mm long), softer segment at the distal end of themagnetically active portion. This softer segment was composed of thePDMS+NdFeB composite only (no inner core 3) and, thus, was substantiallysofter than the remainder which contained the stiff nitinol inner core 3(diameter of 80 μm). The effective Young's modulus of the stiffersegment (14 MPa) was calculated to be 10 times that of the softersegment (1.4 MPa) from Eq. (5) in Materials and Methods section. Bothsegments had uniform magnetization (M=128 kA/m) along the axialdirection. The softer and hence more responsive tip enables creatingmultiple modes and degrees of bending depending on the direction andstrength of the applied actuating field, as well as the unconstrainedlength of the magnetically active segment, as predicted from themodel-based simulation in FIGS. 8A-C. When the unconstrained length ofthe stiff segment equals that of the soft segment, only the very end tipof the continuum robotic device 1 reacts effectively to the appliedmagnetic fields, creating a J-shaped tip (FIG. 8A). This is because theshort unconstrained segment has a large bending stiffness due to thesmall aspect ratio, as predicted in FIG. 16E. As the unconstrainedlength increases, the bending stiffness of the stiffer segmentdecreases, which increases the radius of curvature of overall bendingupon magnetic actuation (FIG. 8B-C). FIG. 8D shows the experimentaldemonstration of our fabricated prototype navigating through a tortuouspath formed by a series of tightly spaced rings (see FIG. 17B fordetails) based on the ability to make sharp turns, which is enabled bythe unique design of the present invention device 1.

In order to provide highly adaptable and smooth navigation throughnonlinear branches of the vasculature, such as the cerebrovasculature(see FIG. 2C-D) which includes very narrow and acute-angled corners, thepresent invention provides a soft continuum robotic device 1 havingomnidirectional steerability based on tether-free actuation mechanismsas well as sufficient mechanical rigidity and pushability (i.e., whenadvancing the device through the vasculature, the proximal part ispushed to advance the device forward as a whole). The designed softcontinuum robotic device 1 may open new avenues to teleoperated,minimally invasive robotic surgery for previously inaccessible lesionsor difficult-to-reach areas across the human body (e.g., FIGS. 10A-B),thereby addressing challenges and unmet needs in healthcare. Potentialapplications of such multifunctional soft continuum robots includelaser-assisted treatment of vascular occlusion (blockage due to athrombus) or atherosclerosis (narrowing due to plaque buildup),so-called laser atherectomy. In addition, the device 1 may further beequipped with a miniature CMOS sensor, while having multiple functionalcores for both illumination and imaging. As such, the soft continuumrobotic device 1 may further enable submillimeter-scale endoscopicprocedures such as angioscopy to better diagnose pathologic conditionssuch as embolism or aneurysms in the affected sites of intracranialarteries, which is far less accessible due to the considerably small andtortuous vascular structures.

To illustrate the potential impacts in medical applications, thesteering and navigating capabilities are demonstrated in realistic,clinically relevant environments (See FIGS. 11, 12 and 20A-C). In onedemonstration, the soft continuum robotic device 1 was magneticallycontrolled to navigate through portions of a real-sized vascular phantommodel (made of silicone) for a particular neurovasculature (theso-called the circle of Willis) with multiple aneurysms (localizeddilation of a blood vessel) at different locations. Noticeably, as canbe seen in FIGS. 20A-C, the vascular structures are highly complex andtortuous, involving several acute-angled corners. As depicted in greaterdetail (FIGS. 20A-C), the inner diameter of the silicone vessels alongthe targeted path (from carotid artery to middle cerebral artery FIG.20B) to be navigated by the soft continuum robotic device 1 ranged from2.5 to 7.5 mm, while the sizes of the aneurysms to reach along the pathwere 9 mm (first), 7.5 mm (second), and 5 mm (third) in diameter,respectively (FIG. 20A). The overall distance navigated by the roboticdevice 1 along the targeted path was around 250 mm (FIGS. 20B-C)). Therequired task for the present invention ferromagnetic soft continuumrobotic device 1 was to reach all the aneurysms along the targeted path,while at the same time demonstrating the ability to locate the elongatebody's 2 distal tip inside each aneurysm based on magnetic actuation andsteering capabilities. In addition, direct contact of the elongate body2 with the inner wall of the aneurysms should be avoided, given the factthat the aneurysms have high risk of rupture, which can lead tohemorrhagic stroke. The present invention ferromagnetic soft continuumrobotic device 1 was demonstrated to successfully carry out the requiredtasks in the vascular phantom, which was filled with a blood analoguethat simulated the friction between commercial guidewires and real bloodvessels. Ferromagnetic soft continuum robotic devices 1 with and withouta self-lubricating hydrogel skin were both tested, with the deviceincluding the hydrogel skin substantially reducing the friction actingon the device while going through the first acute-angled corner, thuspreventing unwanted jerky movement of the device.

Among the series of tasks described above, manually controlled passiveguidewires with pre-bent tips may somewhat easily pass the first sharpcorner and follow the constrained path, owing to their highly passiveand compliant nature. At the second sharp corner next to the firstaneurysm, however, there is no continuous path that is narrow enough toeffectively constrain and guide the passive wire because of the largeempty space within the aneurysm. In this scenario, in the presence ofsuch acute angulation in particular, the J-shaped tip becomes no longeruseful due to the limited range the tip can cover. When pushed further,the tip of the passive wire will unavoidably scratch the inner wall ofthe aneurysm, as the wire coils up and travels along the spherical innersurface while applying unnecessary pressure on the wall. This poses ahigh risk of aneurysm rupture, which can lead to hemorrhagic stroke ifthe aneurysm were located in in the cerebral vasculature.

Due largely to these functional limitations inherent in manuallycontrolled guidewires, it is quite challenging and complex to navigatesuch aneurysms with acute angulation with manual devices even aftermultiple reshaping maneuvers to adjust the shape of the tip. As such,the demonstrated capabilities of the present invention soft continuumrobotic device 1 to navigate through acute, highly nonlinear branches ofneurovasculature will facilitate endovascular procedures by enabling theaccess to difficult-to-reach areas.

Further extending the capabilities of the present inventionferromagnetic soft continuum robotic device 1, additionalfunctionalities enabled by a functional core were further demonstrated.As an illustrative example, an optical fiber was incorporated in theelongate body 2 as the inner core 3 to demonstrate magneticallysteerable laser delivery (FIGS. 6A-B). In the experimental settings forthe demonstration, the fabricated soft continuum robotic device 1 outerdiameter was designed to be 500 μm. The incorporated optical fiber hadan outer diameter of 245 μm and comprised a silica core, cladding, andprotective acrylate coating. The given task was to accurately point atthe small targets (2-mm dots) with the laser beam in a desired orderbased on the magnetic actuation (FIG. 6B). The omnidirectional steeringand the flexible motion allowed the soft continuum robotic device 1 tosuccessfully carry out the desired task.

One potential medical applications of the demonstrated capabilities inFIG. 6 is the laser-assisted treatment of vascular stenosis (oratherosclerosis; narrowing of an artery due to plaque buildup on theinner walls), which commonly occurs in the carotid artery, through whichthe blood is supplied from the heart to the brain. As demonstratedwithin this context, the present invention soft continuum robotic device1 having an incorporated laser-delivering functional inner core 3 wasused in a carotid artery phantom (FIGS. 12A-B). In the demonstration,the device first reached the target site in the carotid artery and thenemitted a laser beam near the inner wall. It then changed the directionand position of the laser-emitting tip using magnetic steering (FIG.12A). After that, it turned off the laser and navigated downstreamthrough the carotid artery (FIG. 12B).

The magnetic steerability and the resulting capability to keep thelaser-emitting tip in position may help preventing unwanted movement ordisplacement of the tip from the desired location during laser ablation,thereby improving the accuracy and the safety, which are of paramountimportance throughout the whole procedure.

By providing the present devices so as to include the above-describedproperties, patients undergoing a procedure will benefit from thereduced time and improved accuracy thus provided, while surgeons willalso benefit from the reduced fatigue and the ability to work away fromthe radiation source required for real-time imaging during theintervention. Further, the present device and method will provide newavenues to telerobotic endovascular neurosurgery to enable earlyintervention of acute ischemic stroke, thereby addressing the currentkey challenges and unmet needs in healthcare.

Materials and Methods

In the various prototypes and examples, the following materials andmethods were used unless otherwise noted.

Ferromagnetic Composite Ink Preparation

The ferromagnetic composite ink was prepared by homogeneously mixingNdFeB microparticles with an average size of 5 μm (MQFP-B-2007609-089,Magnequench) into uncured PDMS resin (Sylgard 184, Dow Corning) or TPU(Elastollan Soft 35A 12P, BASF) dissolved (50 wt %) inN,N-dimethylformamide (Sigma Aldrich) at prescribed volume fractionusing a planetary mixer (AR-100, Thinky) at 2,000 r.p.m. for 2 min. ForPDMS-based ink, 5 wt % of curing agent containing platinum catalyst wasadded in the subsequent mixing for 45 sec under the same condition,after cooling down at room temperature for 1 min. The mixture was thenmagnetized by impulse magnetic fields (about 2.7 T) generated by animpulse magnetizer (IM-10-30, ASC Scientific) to impart magneticpolarities to the NdFeB particles embedded in the unsolidified elastomerresin.

Magnetic Characterization

The magnetic moment densities of ferromagnetic soft composites based onPDMS+NdFeB with different particle concentrations were measured with avibrating sample magnetometer (DMS 1660, ADE Technologies). Specimenswere prepared from thin sheets of the composite materials obtained frommolding by cutting them into 6-mm circles using a biopsy punch (MiltexInc.) to fit into the sample holder of the magnetometer. The remnantmagnetic moments of the samples were measured when the applied externalfield is zero, and then divided by the sample volume to obtain themagnetization, or magnetic moment density.

Mechanical Testing

Rectangular planar sheets (12 mm×35 mm×1 mm) of ferromagnetic softcomposites based on PDMS+NdFeB with different particle concentrationswere prepared by molding and then cut into dog-bone-shaped specimenswith known dimensions (width 4 mm, gauge length 17 mm) for tensiletesting. The specimens were tested on a mechanical testing machine(Z2.5, Zwick/Roell) with a 20-N load cell at a strain rate of 0.01 s⁻¹.Nominal stress-stretch curve was plotted for each specimen, and theshear modulus was identified by fitting the experimental curve to aneo-Hookean model. When compared with the particle-filled elastomers,hydrogels are orders of magnitude softer in terms of Young's modulus.Due to this significantly lower modulus, the hydrogel skin does notcontribute to the bulk mechanical property of the coated specimen.Therefore, for simplicity, the mechanical properties of theferromagnetic soft composites were measured from uncoated sampleswithout hydrogel skins.

Silica Coating of Magnetic Particles

The NdFeB microparticles were coated with a layer of silica (SiO2)through hydrolysis and polycondensation of tetraethyl orthosilicate(TEOS; Sigma Aldrich), widely known as the Stober method, followed bythe nucleation of the silica around the particle. First, 40 g of NdFeBmicroparticles were dispersed in 1,000 mL of ethanol while vigorouslystirring to avoid sedimentation at 1,500 r.p.m using a digital mixer(Cole-Parmer). Then 60 mL of 29% ammonium hydroxide was slowly added tothe mixture, followed by slow addition of 2 mL of TEOS. The mixture wasstirred for 12 hr at room temperature and then washed with acetonemultiple times after the reaction. The suspension was thenvacuum-filtered to obtain the silica-coated particles.

Fabrication of Ferromagnetic Soft Continuum Robotic Devices

The TPU-based prototype demonstrated in FIG. 9B was fabricated byjoining a printed segment to a commercial guidewire with a TPU jacketand a nitinol core. For the process, the prepared composite ink based onTPU+NdFeB (30 vol %) was first loaded into a syringe barrel and thenmounted to the custom-designed 3D printer based on a Cartesian gantrysystem (AGS1000, Aerotech). A conical nozzle (outlet diameter of 838 μm,Smoothflow Tapered Tip, Nordson EFD) was used to extrude the inks byapplying pressure. The printed TPU composite fiber was thermally weldedto a commercial guidewire with a TPU jacket (ZIPwire™ HydrophilicGuidewire; 810-μm diameter, Boston Scientific) using a heat-shrink tube(inner diameter of 1.02 mm; Nordson Medical), in which the two segmentswere placed and locally heated at 190° C. During the heating localizedat the junction, the TPU composites of both segments partially melt andthen join together when cooled down, creating a seamless connection ofthe two segments.

The ferromagnetic soft continuum robots with functional cores presentedin FIGS. 6, 8, 9, 11 and 12 and were fabricated through injectionmolding, for which micro-tubes made of heat-resistant polymers such aspolytetrafluoroethylene or polyimide (Nordson Medical) were used asmolds. For the prototypes presented in FIGS. 8, 9 and 11, in whichtapered nitinol cores were incorporated, commercial guidewire products(Headliner® Hydrophilic Guidewire; 300-μm diameter, Terumo) were used astemplates. For a specified length of the commercial guidewire (25 cmfrom the distal tip), the TPU-based polymer jacket was first partiallymelt and stripped off by locally applying heat (250-300° C.) to exposethe tapered nitinol wire (distal diameter of 80 μm). Then, the preparedferromagnetic composite ink based on PDMS+NdFeB (20 vol %) was injectedinto the 610-μm polyimide micro-tube through a conical nozzle (outletdiameter of 120 μm) while placing the tapered nitinol wire inside themold (FIG. 7B). After curing upon heating at 160-190° C. for 5 min, themold tubing was stripped off with a razor blade to retrieve solidifiedbody of the fabricated ferromagnetic soft continuum robot. For theprototype presented in FIGS. 6 and 12, a single-mode optical fiber(diameter of 245 μm including the 3-μm core, 125-μm cladding, andprotective acrylic polymer coating; Thorlabs Inc.) was used as afunctional core, followed by the same injection molding procedures with510-μm polyimide mold described above.

After growing hydrogel skin on the outer surface (see below), all thedemonstrated prototypes were uniformly magnetized along the axialdirection by applying an impulse magnetic field (about 2.7 T) generatedby an impulse magnetizer (IM-10-30, ASC Scientific) to magneticallysaturate the embedded NdFeB particles.

Hydrogel Skin Formation

Following the previously reported protocol (Y. Yu, H. Yuk, G. A. Parada,Y. Wu, X. Liu, C. S. Nabzdyk, K. Youcef-Toumi, J. Zang, X. Zhao,Multifunctional “hydrogel skins” on diverse polymers with arbitraryshapes, Adv. Mater. 31, 1807101 (2019).), the uncoated samples werefirst cleaned with ethanol and isopropanol, followed by drying undernitrogen flow. To promote wettability of the uncoated polymer, thesamples were treated with a plasma cleaner (PDC-001, Harrick Plasma) for1.5 min. The plasma-treated samples were then immersed in an organicsolution of ethanol containing 10 wt % benzophenone for 10-15 min. Afterremoving excess solution on the surface with wipes, the samples werethen immersed in a pre-gel solution containing 30 wt % hydrogel monomers(N,N-dimethylacrylamide; DMAA, Sigma Aldrich) and 1 wt % Irgacure-2959(Sigma Aldrich) based on deionized water, which was degassed for 5 minbefore preparing the pre-gel solution. For UV curing, the pre-gelsolution bath was subjected to UV irradiation (CL-1000, UVP) for 60 min.Then, unreacted regents were removed by rinsing with deionized waterusing an orbital shaker (Micro Plate Shaker, VWR) for 24 h. For imaginghydrogel skin (FIGS. 15A-D), both coated and uncoated samples (20 mm×20mm×1 mm) based on PDMS+NdFeB (20 vol %) prepared by molding wereimmersed in an aqueous fluorescein solution before imaging to visualizethe hydrogel layer.

Friction Coefficient Measurement

For friction coefficient measurement, both coated and uncoated samples(20 mm×20 mm×1 mm) were prepared based on PDMS+NdFeB (20 vol %)composites. To quantify the friction coefficients, the torque requiredto shear the specimens at prescribed shear rates (from 0.1 to 1.0 s−1)under prescribed normal pressure (from 3 to 9 kPa) was measured from arotational rheometer (AR-G2, TA Instruments) in normal force controlmode with a 20-mm diameter steel plate geometry. Deionized water wassmeared on top of both coated and uncoated surfaces before shearing thesamples. The friction coefficients were calculated following thepreviously reported protocol (G. A. Parada, H. Yuk, X. Liu, A. J. Hsieh,X. Zhao, Impermeable robust hydrogels via hybrid lamination, Adv.Healthc. Mater. 6, 1700520 (2017).).

Pulling Test

For pulling test, a cylindrical geometry was adopted for relevance tothe actual shape of the continuum robot. For ease of measurement,however, the pulling test was conducted with a large-scale prototypewith 8-mm diameter. Both coated and uncoated sheets of PDMS+NdFeB (20vol %) composites with 1-mm thickness were wrapped around a glass rodwith silicone adhesives (Sil-Poxy, Smooth-On Inc.) applied for securebonding. The normal force was applied by a pair of half-cylindricalgrips made of PDMS, where the upper grip was connected to the load cellof a mechanical testing machine (Z2.5, Zwick/Roell) to apply theprescribed normal force (2 N or 5 N) to the specimen held between thegrips. The specimens were pulled by a linear actuator equipped with aload cell at a constant speed (200 mm/min) while being immersed in abath of deionized water. The load cell attached on the pulling gripmeasured the forces required to pull the specimens at the given speed.

Finite Element Analysis

For simulation results presented in FIGS. 8 and 16, a user-definedelement subroutine was used with the commercial finite-element analysissoftware Abaqus. For parametric studies presented in FIGS. 16F, H and I,Eqs. (2) and (3) were implemented as input parameters for materialproperties. In all simulations, the bulk modulus was set to be 1,000times the shear modulus to approximate the incompressibility, and thedirection and strength of a uniform magnetic field were defined asadditional input parameters. For simulating multiple modes and degreesof bending presented in FIG. 8A-C, the magnetization parameter wasdefined as M=128 kA/m, which was experimentally measured for samples ofPDMS+NdFeB (20 vol %) composite. For the mechanical property of thesofter segment, which is composed of the ferromagnetic elastomercomposite only, the shear modulus value (G=455.6 kPa) measured forPDMS+NdFeB (20 vol %) was used. Under the assumption of incompressiblesolids, for which E=3G, this shear modulus value is translated toYoung's modulus of 1366.8 kPa. For the stiffer segment, which contains a80-μm nitinol wire, the effective Young's modulus (E_(eff)=14008 kPa)was calculated from the following relation:

$\begin{matrix}{E_{eff} = ( {{E_{core}( \frac{d}{D} )}^{4} + {E_{jacket}( {1 - ( \frac{d}{D} )^{4}} )}} )} & (5)\end{matrix}$where E_(core) and E_(jacket) denote the Young's moduli of the nitinolcore and the ferromagnetic soft polymer jacket, respectively, while dand D denote the core and jacket diameter. For the nitinol core, Young'smodulus of the martensite phase (40 GPa) was used for the calculation.This calculation was validated by the good agreement between thesimulation and experimental results presented in FIG. 20E.

Experimental Validation of Simulation Results

For experimental validation of the model-based simulation resultspresented in FIG. 20E, deflection angles were measured from circularbeam specimens (diameter of 600 μm) with different aspect ratios(L/D=10, 15, 20) under uniform magnetic fields ranging from 5 to 60 mTgenerated by a pair of Helmholtz coils (10-cm diameter; MicroMagnetics).For each specimen, a nitinol core (diameter of 80 μm) was incorporatedin the ferromagnetic soft polymer jacket based on PDMS+NdFeB (20 vol %)through the injection molding process described earlier to providesufficient mechanical stiffness required to prevent gravitationalsagging during the measurement. The incorporation of the stiff coreresults in ten-fold increase in the Young's modulus (and therefore thebending stiffness as well) by 10 times as described above.

Magnetic Actuation and Demonstration

For all demonstrations presented herein, unless otherwise noted, acylindrical NdFeB magnet (diameter and height of 50 mm; DY0Y0-N52, K&JMagnetic, Inc.) was used to apply magnetic fields required for actuationat distance. For magnetic steering and navigation, the direction andstrength of the applied magnetic fields were varied by manuallymanipulating the magnet to change its position and orientation whileadvancing the proximal end of the template guidewire connected to thedemonstrated ferromagnetic soft continuum robot. Detailed dimension ofthe demonstration setups in FIGS. 8 and 9, such as the location, height,and tilting angle of the rings, is provided in FIG. 17.

For demonstrations presented in FIGS. 11 and 12, a commerciallyavailable cerebrovascular phantom model made of silicone (Trandomed 3D)was used along with a blood-mimicking fluid (Replicator Fluid, VascularSimulations Inc.), which simulates the friction between commercialhydrophilic guidewire/catheter surfaces and the real blood vessels whenused with a silicone vascular model. Further details on thecerebrovascular phantom model is provided in FIG. 20. In these set ofdemonstrations, the steering and navigating tasks were controlled withvisual feedback by manually manipulating the position and orientation ofa single permanent magnet while advancing the whole body by pushing theproximal end. The magnetic manipulation under spatially non-uniformfields exploited the magnetic body torques as the main source ofactuation as well as the magnetic body forces, which further help therobot's tip to align itself towards the desired direction moreeffectively (more detailed discussions on the actuation mechanisms areavailable in Supplementary Text). For steerable laser deliverydemonstration presented in FIGS. 6 and 12, a 3,100-mW green LED (530-nmwavelength; Thorlabs Inc.) was used as a light source.

Optimization Design

Both magnetic and mechanical properties of the robot's body made offerromagnetic soft composites vary with the particle loadingconcentration. A material design strategy is described here to optimizethe actuation performance of the present invention ferromagnetic softcontinuum robotic device. Based on the theoretical framework developedfor ferromagnetic soft materials (Y. Kim, H. Yuk, R. Zhao, S. A.Chester, X. Zhao, Printing ferromagnetic domains for untetheredfast-transforming soft materials, Nature 558, 274-279 (2018); R. Zhao,Y. Kim, S. A. Chester, P. Sharma, X. Zhao, Mechanics of hard-magneticsoft material, J. Mech. Phys. Solids 124, 244-263 (2019).), we firstprovide the fundamental equations for quantitative description of thedeformation of ferromagnetic soft materials upon magnetic actuation. Themagnetic moment density (or magnetization) at any point of aferromagnetic soft material in the reference (undeformed) configurationis denoted by a vector M. Under an applied magnetic field, denoted by avector B, the ferromagnetic soft material can deform. The deformation atany point of the material is characterized by the deformation gradienttensor F. The application of the magnetic field on the embedded magneticmoment in the material generates the magnetic Cauchy stress that drivesσ^(magnetic)=B⊗FM the deformation, where the operation ⊗ denotes thedyadic product which takes two vectors to yield a second order tensor.Meanwhile, the deformation of the material generates the elastic Cauchystress σ^(elastic), which is also a function of F defined byhyperelastic constitutive models such as the neo-Hookean model. Thetotal Cauchy stress in the material σ=σ^(elastic)+σ^(magnetic) is thensubstituted into the equilibrium equation in Eq. (S3) (See SupplementaryText), from which the deformation (i.e. F) can be evaluated at everymaterial point in equilibrium. While alternate approaches based onmagnetic body forces and torques have been proposed to calculate thedeformation of ferromagnetic soft materials, the current approach basedon the magnetic stress can be readily implemented in commercialfinite-element software packages such as Abaqus. In addition, themagnetic stress can readily recover the magnetic body force and torquedensities used in other approaches (see Supplementary Text for details).

Since the magnetically responsive tip of the present invention softcontinuum robotic device is axially magnetized (i.e. M along the axialdirection), the tip tends to bend along the applied magnetic field B(FIG. 5B) due to the magnetic body torques generated from the embeddedmagnetized particles. To find the optimal particle concentration thatyields the largest bending under given conditions and geometry, withoutloss of generality, we consider a beam of length L and diameter D underuniform magnetic field B that is being applied perpendicularly to M(FIG. 16A). Also, to utilize a tractable analytical solution, we furtherassume that the magnetically active tip undergoes small bending, wherethe deflection (denoted in FIG. 16A) is below 10% of the tip length L.Then, we can reach the following analytical expression for thedeflection of the magnetically active tip (details are available inSupplementary Text):

$\begin{matrix}{\frac{\delta}{L} = {\frac{16}{9}( \frac{MB}{G} )( \frac{L}{D} )^{2}}} & (1)\end{matrix}$where M and B are the magnitudes of the magnetization and the appliedmagnetic field, respectively, and G denotes the shear modulus of thematerial, which is considered as a neo-Hookean solid in the currentanalysis. Eq. (1) relates the material properties (magnetization M andshear modulus G), geometry (beam length L and diameter D), and actuatingfield strength B to the normalized deflection. From Eq. (1), it can bededucted that for small bending, the deflection of the beam is linearlyproportional to a dimensionless quantity MB/G while quadraticallydependent on the aspect ratio L/D. The dimensionless quantity MB/G canbe interpreted as the actuating field strength normalized by thematerial properties. Given that both M and G are dependent on theparticle volume fraction, Eq. (1) implies that there will likely be anoptimal point at which the normalized deflection is maximized.

The magnetization of the ferromagnetic soft composite is linearlyproportional to the volume fraction of NdFeB particles (FIG. 16B), andhence can be expressed asM=M _(p)ϕ  (2)where M_(p) denotes the magnetization of the magnetic particles, and ϕdenotes the particle volume fraction. Unlike the magnetization, theshear modulus increases nonlinearly as the particle concentrationincreases (FIG. 16C). This nonlinear dependence of shear modulus can bepredicted by a simple analytical expression in Eq. (3), so-called aMooney model, under the assumption that the increase in the shearmodulus of particle-filled elastomer composites is analogous to theincrease in the viscosity of particle suspensions.

$\begin{matrix}{G = {G_{o}\mspace{14mu}{\exp( \frac{2.5\phi}{1 - {1.35\phi}} )}}} & (3)\end{matrix}$where G_(o) denotes the shear modulus of a pure elastomer with noparticle. No significant difference is observed in both magnetization(FIG. 16B) and shear modulus (FIG. 16C) between the composite based onuncoated particles (PDMS+NdFeB) and the composite based on silica-coatedparticles (PDMS+NdFeB@SiO₂), which may be attributed to the marginalchange in the particle volume due to the marginal thickness of thesilica shell as discussed earlier. The small difference in shear modulusbetween the two types of composites also implies that the affinities ofsilicone elastomers with metal oxide (of uncoated particles) and siliconoxide (of silica-coated particles) surfaces are not substantiallydifferent.

By substituting Eqs. (2) and (3) into Eq. (1), the criticalconcentration at which the deflection is maximized for given conditions(field strength B and the geometric factor L/D) can be identified. Thecritical volume fraction is calculated to be 0.207 (or 20.7 vol %),independent of M_(p) and G_(o) (FIG. 16D). It should be noted that thiscritical concentration is obtained for small bending scenarios asdescribed above. For large bending, the simulation and experimentalresults in FIG. 16E indicate that the actuation angle (defined in FIG.16A) monotonically increases as a function of the normalized fieldstrength MB/G for different aspect ratios L/D. Therefore, it isanticipated that the critical concentration predicted from thesmall-deflection analysis will remain effective for large bending casesas well. This is further validated by the simulation results for largebending presented in FIG. 16F, which show the actuation angle varyingwith the material composition and the applied field strength for a fixedgeometry. The results clearly indicate that the actuation angle reachesits maximum, for given applied field strengths, at the critical volumefraction (20.7 vol %) predicted above for small bending case. As theapplied field strength increases, however, the actuation angle begins tosaturate while approaching 90 degrees, making the curves around the peakflat (FIG. 16F).

When producing mechanical work out of magnetic actuation is of greaterimportance than the large deflection, the actuation performance can beoptimized in terms of the energy density, which corresponds to theamount of work (per unit volume) one can extract from the continuumrobotic device. For small bending, the equivalent force generated at thefree end of the beam can be calculated as F=MBA (see SupplementaryText), where A denotes the cross-sectional area of the beam. Combiningthis with Eq. (1), an analytical expression for the energy density u isas follows:

$\begin{matrix}{u = {\frac{16}{9}( \frac{M^{2}B^{2}}{G} )( \frac{L}{D} )^{2}}} & (4)\end{matrix}$By substituting Eqs. (2) and (3) into Eq. (4), it can be deducted thatthe energy density reaches its maximum when the particle volume fractionis 29.3 vol % under given conditions in terms of applied field strengthB and geometry L/D (FIG. 16G). This analytical prediction is validatedby our model-based simulation for small bending (FIG. 16H), which showshow the energy density varies with the particle volume fraction when B=5mT and L/D=10. As the bending becomes larger, however, the peak at whichthe energy density is maximized shifts to the right, towards the highervolume fractions (FIG. 16I). The peak eventually disappears when theactuation angle saturates, after which the energy density keepsincreasing with the particle volume fraction. Qualitatively, this can beunderstood by considering the exponentially increasing stiffness (FIG.16C), which dominantly contributes to the energy density when thedeformation level remains almost unchanged (FIG. 16F).

When the material properties M and G are fixed due to a given particlevolume fraction, the actuation performance of the present inventionferromagnetic soft continuum robotic device under given applied fieldstrength can still be optimized by adjusting the aspect ratio, accordingto Eqs. (1) and (4) along with the simulation results presented in FIG.16. This implies that fine features with high aspect ratios, such ascilia-like soft continuum robots, would require significantly lowerfield strength to induce the bending actuation. Given that aprinting-based method can easily produce very fine features, down to 80μm in terms of diameter, such extremely thin, cilia-like soft continuumrobots may also be designed for applications that require manipulatinghighly delicate structures.

Supplementary Text

Analytical Model for Ferromagnetic Soft Continuum Robots

We denote the magnetic moment density (or magnetization) at any point ofa ferromagnetic soft material in the undeformed, reference configurationby a vector M. Under an applied magnetic field, denoted by a vector B,the ferromagnetic soft material deforms (FIG. 16A). The deformation atany point of the material is characterized by the deformation gradienttensor F. The application of the magnetic field on the embedded magneticmoment in the material generates the magnetic Cauchy stress that drivesits deformation. Meanwhile, the deformation of the material generatesthe elastic Cauchy stress. For incompressible solids, the magneticCauchy stress can be expressed asσ^(magnetic) =−B⊗FM,  (S1)where the operator ⊗ denotes the dyadic product, which takes two vectorsto yield a second-order tensor. If adopting the incompressibleneo-Hookean model among existing hyperelastic constitutive models, theelastic Cauchy stress σ^(elastic) can be expressed asσ^(elastic) =GFF ^(T) −p1,  (S2)where G is the shear modulus, F^(T) is the transpose of F, 1 is theidentity tensor, and p is the hydrostatic pressure that needs to bedetermined from boundary conditions. Assuming a quasi-static processwhere the inertial effects are negligible, the total Cauchy stressσ=(σ^(elastic)+σ^(magnetic) can then be substituted into the followingequilibrium equation:div σb=0,  (S3)where div denotes the divergence, and b denotes the body force (e.g.,gravitational) per unit volume. This equilibrium equation can be solvedto calculate the deformation gradient F, thereby finding the equilibriumconfiguration of the ferromagnetic soft material under magneticactuation. While alternate approaches based on magnetic forces andtorques have been developed to calculate the deformation offerromagnetic soft materials, the current approach based on magneticstress can be readily implemented in commercial finite-element softwarepackages such as Abaqus.

For the magnetically responsive tip of the present inventionferromagnetic soft continuum robotic device, the gravitational bodyforce can be neglected due to the considerably greater contribution fromthe magnetic stress. For small bending, where the deflection of the freeend is below 10% of the length, i.e., (δ/L≤0.1, a tractable analyticalexpression can be derived for quantitative prediction of the degree ofdeflection. When a uniform magnetic field B is applied perpendicularlyto the magnetically responsive tip with magnetization M along the axialdirection (FIG. 3A), the only non-zero, shear stress component of themagnetic Cauchy stress tensor in the reference (undeformed)configuration (i.e., F=1) can be calculated from eq. S1 to beσ^(magnetic)=−MB as a scalar quantity. This shear stress gives rise tomagnetic moment T=−MBAL across the body, where A denotes thecross-sectional area (A=πD²/4) of the circular beam of length L anddiameter D. Equivalently, this magnetic moment can be considered to begenerating a point force F=MBA at the free end of the beam along theapplied field direction (as illustrated in the schematic in FIG. 6G).The bending stiffness of a beam can be expressed as K_(b)=3EI/L³, whereE denotes the Young's modulus of the constituent material, and I denotesthe area moment of inertia, which can be expressed as I=πD⁴/64 for acircular beam of diameter D. Then, the deflection of the free end of thebeam, denoted δ, under this point force can be analytically expressedfor small deflection case as

$\begin{matrix}{\delta = {\frac{F}{K_{b}} = {\frac{16{MBL}^{3}}{3{ED}^{2}}.}}} & ({S4})\end{matrix}$For incompressible solids, for which E=3G, eq. S4 can be expressed in anormalized from as

$\begin{matrix}{{\frac{\delta}{L} = {\frac{16}{9}( \frac{MB}{G} )( \frac{L}{D} )^{2}}},} & ({S5})\end{matrix}$which is identical to Eq. 1 introduced in the main text. From thisanalytical expression, it becomes evident that the normalized deflectionof the magnetically responsive tip of the ferromagnetic soft continuumrobot under magnetic actuation is determined by the two dimensionlessfactors: MB/G and L/D. The former can be interpreted as the appliedfield strength B normalized by the material properties M and G, whilethe latter is the aspect ratio, a geometric factor that can largelyaffect the bending stiffness of the beam. Also, the work done by theequivalent point force F while deforming the beam by small deflection δ,can be expressed as

$\begin{matrix}{W = {{F\;\delta} = {\frac{4\pi}{9}( \frac{M^{2}B^{2}}{G} ){L^{3}.}}}} & ({S6})\end{matrix}$When divided by the volume, eq. S6 leads to the following expression forthe energy density:

$\begin{matrix}{{u = {\frac{16}{9}( \frac{M^{2}B^{2}}{G} )( \frac{L}{D} )^{2}}},} & ({S7})\end{matrix}$which is identical to Eq. 4 introduced in the main text. Here, it isassumed that no energy is dissipated during the deformation, based onthe underlying assumption of hyperelastic solids.

Alternative Model Based on Magnetic Body Force and Torque

As discussed in the previous section, the current model forferromagnetic soft materials is based on the magnetic Cauchy stress andthe total Cauchy stress. Since the traditional approaches are commonlybased on the magnetic body force and torque and the elastic Cauchystress, here we describe how the magnetic Cauchy stress presented in eq.S1 is elated to the magnetic body force and torque in the followingparagraphs.

Under the assumption of ideal hard-magnetic soft materials, which holdsfor the present invention ferromagnetic soft continuum robotic device,the magnetization vector M can be considered independent of theexternally applied magnetic field B, provided that the materials aremagnetically saturated and that the applied field strength is far belowthe coercivity, at which the reversal of remnant magnetization takesplace.

For an incompressible ideal hard-magnetic soft material, the magneticbody torque per unit volume of the material (i.e., magnetic body torquedensity) generated by the external field B applied to the magnetizationM can be expressed as a consequence of the angular momentum balance asτ^(magnetic)=−ε:(σ^(magnetic))^(T) =FM×B,  (S8)where ε=ε_(ijk)e_(i)⊗e_(j)⊗e_(k) is the third-order permutation tensor;the operator: denotes the double contraction of two tensors; and FMaccounts for the reoriented magnetization vector in the current(deformed) configuration. Also, the magnetic body force acting on thematerial per unit volume (i.e., magnetic body force density) under theapplied magnetic field B can be expressed from eqs. S1 and S3 asb ^(magnetic)=−div σ^(magnetic)=(gradB)FM,  (S9)where gradB denotes the spatial gradient of the applied magnetic field,which gives rise to the magnetic body force. Again, FM represents thereoriented magnetization vector in the current (deformed) configuration.Note that the magnetic body torque and force densities in eqs. S8 and S9are consistent with their common expressions used elsewhere in theliterature.

As shown above, our approach based on the magnetic Cauchy stress enablesthe magnetic body torques and forces to be treated as stresses whensolving the equilibrium equations to find the final configuration of theferromagnetic soft continuum robotic devices under external magneticfields. When the magnetic body torque and force (instead of the magneticCauchy stress) are directly employed to calculate the deformation, theelastic Cauchy stress (instead of the total Cauchy stress) can be usedin the equilibrium equations. While the models based on the magneticCauchy stress or the magnetic body torque and force will reach the samedeformation, the current approach based on the magnetic Cauchy stresscan be readily implemented in commercial finite-element softwarepackages such as Abaqus and hence enables accurate and quantitativeprediction of the final configuration of the robot under givenconditions and geometries.

Magnetic Actuation and Steering with a Cylindrical Permanent Magnet

For the demonstrations presented earlier, a cylindrical NdFeB magnet(DY0Y0-N52, K&J Magnetic, Inc.) was employed to demonstrate the magneticsteering function of the proposed ferromagnetic soft continuum robot.The radius (25 mm) and height (50 mm) of the cylindrical magnet aredenoted as R and H, respectively (FIG. 18A). The basic principle behindthe magnetic steering is to align the central axis of the magnet along adesired direction to induce the bending actuation on the robot'smagnetically responsive tip towards and along the desired direction(FIG. 18B). The degree of bending, which is determined by the appliedfield strength, is controlled by adjusting the distance between themagnet and the robot. The magnetic field strength along the central axisof the magnet can be expressed as a function of distance from thesurface (denoted z in FIG. 18A) as

$\begin{matrix}{{{B_{z}( {0,0,z} )} = {\frac{B_{0}\sqrt{R^{2} + H^{2}}}{H}( {\frac{z + H}{\sqrt{( {z + H} )^{2} + R^{2}}} - \frac{z}{\sqrt{z^{2} + R^{2}}}} )}},} & ({S10})\end{matrix}$where B₀ denotes the surface field strength, which is 662 mT for thespecific magnet used. Typical working distance for the set ofdemonstrations presented earlier ranges from around 40 to 80 mm, whichcorresponds to the field strength ranging from around 20 to 80 mT,according to eq. S10. As discussed earlier, under spatially uniformmagnetic fields, the bending actuation of the ferromagnetic softcontinuum robot is driven by magnetic body torques generated from theembedded magnetic particles. However, when the applied actuating fieldis spatially nonuniform, which is the case when a single permanentmagnet is used for actuation, magnetic body force does exist and cancontribute to the magnetic steering and control of the robot.

Influence of a FieldGgradient on the Magnetic Actuation and Steering

When the magnet used for actuation is sufficiently larger than theresponsive segment of the robot being actuated, we may neglect thenonlinear edge effects and further assume that the magnetic fields arealmost parallel and that their spatial gradients are dominant along thecentral axis (z-direction) in terms of the magnitude, as illustrated inFIG. 19. Then, the field gradient along the central axis of thecylindrical magnet discussed above can be obtained from eq. S10 as

$\begin{matrix}{\frac{\partial B_{z}}{\partial z} = {\frac{B_{0}\sqrt{R^{2} + H^{2}}}{H}{( {\frac{1}{\sqrt{( {z + H} )^{2} + R^{2}}} - \frac{1}{\sqrt{z^{2} + R^{2}}} + \frac{z^{2}}{( {z^{2} + R^{2}} )^{3\text{/}2}} - \frac{( {z + H} )^{2}}{( {( {z + H} )^{2} + R^{2}} )^{3\text{/}2}}} ).}}} & ({S11})\end{matrix}$This gives the gradient values ranging from around 0.5 to 4 mT/mm overthe range of typical working distance (40 to 80 mm) mentioned above.Under this range of field gradients, the magnetic body force per unitvolume can be estimated from eq. S9 to be on the order of 64 to 512kN/m³, when assuming that the robot's tip is aligned with the centralaxis of the magnet as illustrated in FIG. 19. Compared to this, thegravitational body force per unit volume is calculated to be on theorder of 23 kN/m³ for ferromagnetic soft composite based on PDMS+NdFeB(20 vol %), whose mass density is 2.273 g/cm³. This means that the fieldgradient along the central axis of the magnet is generating sufficientmagnetic body forces in the deformed configuration to maintain therobot's deflected tip, as illustrated in FIG. 19.

As eq. S9 implies, however, the magnetic body force varies with theconfiguration, or more specifically, the alignment between the robot'smagnetization and the external field. In the reference configuration inFIG. 19, in which the external field is being applied perpendicularly tothe magnetization vector, the magnetic body force acting along themagnet's central axis is considered to be almost negligible. As the bodydeforms and the robot's magnetization becomes more aligned with thecentral axis of the magnet, the magnetic body force increases.Therefore, we can conclude that, when the robot is actuated andcontrolled with a magnet, the bending actuation in general is initiatedand driven by the magnetic body torque and then facilitated and furthersupported by the magnetic body force as the robot's elongate bodydeforms. We therefore can also conclude that utilizing spatial gradientsto exploit magnetic body forces for bending actuation can be a goodstrategy to more effectively control the robot's configurations, asdemonstrated in the series of functional tasks presented earlier.

What is claimed is:
 1. A continuum robotic device for use in minimallyinvasive procedures comprising: an elongate body having a proximal end,a distal end, an inner core and an outer shell; the outer shellfabricated of an elastomer; a plurality of ferromagnetic microparticlesdispersed within the outer shell; the elongate body having an initialshape; wherein the elongate body has an outer diameter of less than 1000μm, and wherein exposure of the device to an external magnetic fieldmagnetically activates the plurality of ferromagnetic microparticles toprovide the elongate body in an activated shape different than theinitial shape.
 2. The device of claim 1, wherein exposure of the deviceto an external magnetic field magnetically activates the plurality offerromagnetic microparticles to provide omnidirectional steering of thedevice.
 3. The device of claim 1, wherein the plurality of ferromagneticmicroparticles are uniformly magnetized along an axial direction of theelongate body.
 4. The device of claim 1, wherein the plurality offerromagnetic microparticles are magnetized in nonuniform patterns ofmagnetic polarity along an axial direction of the elongate body.
 5. Thedevice of claim 1, wherein the plurality of ferromagnetic microparticlesare magnetized in patterns of magnetic polarity comprising alternatingpatterns of different magnetic polarities.
 6. The device of claim 5,wherein alternating patterns of different magnetic polarities aredisposed such that exposure of the device to an external magnetic fieldmagnetically activates the plurality of ferromagnetic microparticles toprovide the elongate body in a wavy shape.
 7. The device of claim 1,wherein the inner core and outer shell and fabricated, and the pluralityof ferromagnetic microparticles are dispersed, such that exposure of thedevice to various direction and magnitudes of external magnetic fieldsprovides the body in a variety of activated shapes different than theinitial shape.
 8. The device of claim 1, wherein the elongate body hasan outer diameter no greater than 900 μm.
 9. The device of claim 1,further comprising a hydrogel skin disposed on the outer shell.
 10. Thedevice of claim 9, wherein the hydrogel skin has a thickness of 10 μm to25 μm.
 11. The device of claim 1, wherein the ferromagneticmicroparticles have an average particle size of 2 μm to 10 μm.
 12. Thedevice of claim 1, wherein the inner core is a metallic wire.
 13. Thedevice of claim 12, wherein the metallic wire is fabricated ofsuperelastic nickel-titanium, stainless steel, platinum aplatinum-tungsten alloy, a cobalt-chromium-molybdenum alloy, orcombinations thereof.
 14. The device of claim 1, wherein the inner corecomprises one or more optical fibers.
 15. The device of claim 1, whereinthe elongate body further includes one or more imaging, illumination,laser delivery, or sensing elements at or near the distal end.
 16. Thedevice of claim 14, wherein the inner core is a fiber optic shapesensor.
 17. The device of claim 16, wherein the fiber optic shape sensorcomprises one or more Bragg grating.
 18. The device of claim 1, whereinthe outer shell is fabricated of one or more polymer materials having aYoung's modulus below 20 MPa.
 19. The device of claim 18, wherein theouter shell is fabricated of an elastomer.
 20. The device of claim 18,herein the outer shell is fabricated of one or more materials selectedfrom natural rubbers, synthetic rubbers, and thermoplastic elastomers.21. The device of claim 20, wherein the outer shell is fabricated of oneor more materials selected from silicone rubber, polyacrylate rubber,thermoplastic polyurethane, and styrene-ethylene-butylene-styrene(SEBS).
 22. The device of claim 1, wherein the plurality offerromagnetic microparticles are selected from neodymium iron boron(NdFeB), samarium cobalt (SmCo), aluminum-nickel-cobalt (AlNiCo),copper-nickel-iron (CuNiFe), barium-iron oxide (BaFeO), platinum-cobaltalloys, and combinations thereof.
 23. The device of claim 1, wherein theferromagnetic microparticles have programmed magnetic polarities thatenable magnetic actuation upon exposure to an external magnetic field.24. The device of claim 1, wherein the plurality of ferromagneticmicroparticles are coated with a non-corrosive layer.
 25. The device ofclaim 24, wherein the non-corrosive layer is fabricated of one or morematerials selected from silica, Parylene C, gold, and epoxies.
 26. Thedevice of claim 1, wherein the inner core extends through a portion ofthe elongate body that is less than an entire length of the elongatebody.
 27. The device of claim 26, wherein a portion of the elongate bodyat the distal end does not include the inner core.
 28. The device ofclaim 1, wherein the inner core tapers in diameter towards the distalend of the elongate body.
 29. The device of claim 1, wherein theelongate body comprises one or more magnetically active portionscomprising one or more outer shell portions containing a plurality offerromagnetic particles dispersed therein, and one or more inactiveportions comprising one or more one or more outer shell portions notcontaining a plurality of ferromagnetic particles dispersed therein. 30.The device of claim 29, wherein a distal end portion of the elongatebody comprises a distal magnetically active portion, and an adjacentportion of the elongate body proximal the distal magnetically activeportion comprises a magnetically inactive portion.
 31. A method ofperforming a minimally invasive procedures on the microvascular systemcomprising: providing a continuum robotic device comprising an elongatebody having a proximal end and a distal end, the elongate body includingan inner core and an outer shell; the outer shell fabricated of anelastomer; a plurality of ferromagnetic microparticles dispersed withinthe outer shell; and the elongate body having an initial shape, whereinthe elongate body has an outer diameter of less than 1000 μm; insertingthe distal end into a blood vessel connected to one or more target sitesof the microvascular system; and actively guiding the distal end andadvancing the elongate body through the microvascular system, includingnonlinear branches of the microvascular system, to the one or moretarget sites using an external magnetic field to activate the pluralityof ferromagnetic microparticles, wherein the external magnetic field isselectively applied to the elongate body, wherein selectively exposingthe elongate body to one or more external magnetic fields is carried outso as to provide the elongate body in a variety of activated shapesconfigured to guide the distal end and advance the elongate body throughmicrovascular system to the one or more target sites.
 32. The method ofclaim 31, wherein selectively exposing the elongate body to one or moreexternal magnetic fields creates multiple controllable modes and degreesof bending of the elongate body depending on a direction and strength ofthe external magnetic field.
 33. The method of claim 31, wherein a userperforms the method remotely by viewing the device within themicrovascular system using real time imaging, and applying the one ormore external magnetic fields using a robotic manipulation platform. 34.The method of claim 31, wherein the minimally invasive procedure is acerebrovascular or endovascular procedures.
 35. The method of claim 31,wherein the one or more target sites are selected from one or moreaneurysms, embolisms, lesions, or arteries.
 36. The method of claim 31,wherein the device inner core comprises one or more optical fiber, andthe device further includes a laser delivery element at the distal end,and the method further comprises guiding and advancing the distal end toone or more target sites selected from vascular occlusions,atherosclerosis, aneurysms, embolisms, and lesions, and treating the oneor more target sites with the laser.
 37. The method of claim 31, whereinthe device inner core comprises one or more optical fiber, and thedevice further includes one or more imaging, sensing, and/orillumination elements, and the method further comprises providingimaging, sensing, and/or illumination while guiding the distal end andadvancing the elongate body through the microvascular system to the oneor more target sites.
 38. The method of claim 31, wherein the deviceinner core comprises one or more fiber optic shape sensors, and themethod further comprises using the one or more fiber optic shape sensorsto provide a user with real-time feedback of a 3D shape of the elongatebody.
 39. The method of claim 38, wherein the one or more fiber opticshape sensors comprise one or more Bragg grating, and the method furthercomprises exposing the one or more Bragg grating to strain ortemperature to shift a wavelength of the Bragg grating, and determininga direction and magnitude of the shift.
 40. A system for performing aminimally invasive procedures on the microvascular system comprising: acontinuum robotic device for use in minimally invasive procedurescomprising: an elongate body having a proximal end, a distal end, aninner core and an outer shell; the outer shell fabricated of anelastomer; a plurality of ferromagnetic microparticles dispersed withinthe outer shell; the elongate body having an initial shape; wherein theelongate body has an outer diameter of less than 1000 μm; and a controlmechanism comprising a single permanent magnet held and manipulated by amulti-degree of freedom (DOF) robotic arm, wherein exposure of thecontinuum robotic device to an external magnetic field from the controlmechanism magnetically activates the plurality of ferromagneticmicroparticles to provide the elongate body in an activated shapedifferent than the initial shape.